Best not to use him again, as hes proven that he cant do it properly.
It is only a small patio at 12 square metres.
The slabs need to be bedded down on a sand/cement mix about 4 to 1 make sure it a coarse sand, similar to floor screed sand.
Bed them down on a full bed of wet mortar, not on dabs.
Sub-base minimum of 100mm type 1 mot well compacted, point up your joints when laid.

In response to your question, your builder should have removed up to 150mm of soil / clay and added a good 100mm sub base and compacted then laid slabs on a good 50mm bed sand / cement wet mix.

hi paul, sorry to hear you were tucked up by getting told you dont need hardcore (sub-base) to be put down. what can i say, best bet post a job on this site, then get a least 3 quotes from paving contractors to rip up the paving, dig down to reach a depth so you have 100mm sub-base (GEO-TEX AREA) then relay the slabs on a semi scread mix or spot them. hope you get your work done for summer.

dont go for the cheapest quote next time ,if this guy had priced right he would have priced to dig out 250mm and stone base up and compact ,this obviously makes the job more expensive but it will be built to last as you have found out to your cost pay cheap pay twice ,only remedy now is to start again lift ,dig and stone up ,compact and relay and point job done properly this time ,let that be a lesson to you ,if its too cheap its tempting but not worth it
regards mitch
substructure

Hi,I would dig a sub base of 150mm +75mm for slabs on mortar,lay 150mm of scalps and compact with small waker plate,lay slabs on a bed of 4 parts sharpe sand to 1 part cement and plasticiser for duerubility,mix untll stodgy,do not dot and dabunder slabs,use a full bed of mortar to prevent rocking slabs.when pointing use semi dry mortar,mix in a little pva glue.

unfortunately some people they believe are the experts for everything so wrong,very wrong,and is not necessary the cheapest quote for the clients,i advice people like a TV but never no one takes me seriously and all the time clients pay for everything,next time when you have something done research your project how to be done properly and after that hire someone only once to do it properly,everyone who is following my advice saves time,money and lots of stress
thanks peter

Get a new builder,advertise your job on this site for some professional advice that is backed by genuine customer feedback.

When you lay any type of Paving you need a good Solid Sub-base.
Crushed concrete or Scalpings. On a driveway we would always put a minimum of 200mm of Scalpings down and compact them. 100mm on a patio.

Clay moves, it shrinks and expands, also known as clay heave.

Before putting the scalpings down you should lay a membrane inbetween the clay and scalpings this will help.

The first thing that should be the base, maybe the second thing he did mix invalid (for 4 bags of sharp sand, half a bag of cement and a little PVA) (This is my private patent!)
I also painted slabs from the bottom of the mix. (PVA + Cement and a little water)
After drying slabs but goes along with the mix.
If this is the driveway is needed is a solid base.

Regards.
Krystian

it should of had at least 100mm of hardcore , and the slabs laid on a full bed of mortar and pointed in

you need a good hardcor base underneath as clay will swell and retract depending on the moisture content, your builder is wrong to suggest otherwise,regards Terry.

you really need to be very carefull when choosing a tradesmen,people who do this sort of work are ten a penny,reccomendation from family or friends is this best way,a good tradesmen will be city and guild qualified ,insured and reliable,with pride in his work,pick somebody with alot of expierence and ask to see his work.

When laying patio slabs, due to the thinness of the size. You are required by rights to place a Sand Base twice the depth of the Slab under the same depth of a Slabbase or Cement Base before you actually go a head and lay these types of Slabs down.

Normal Standard 3x2's or 2x2's, don't need this type of coverage, as there size and weight will hold the Slab in place. Though in saying this...........when I lay 3x2's or 2x2's, I use a dry cement mix over the Sand Base and then a Sand Mortar mix too over the top of the Slabs, thus giving the Slabs a more firm and cohesive strength.

The Cheapest is not always the Best, but 'Value' for 'Money' is what is required overall.

Hi, think you alrady know answer - hardcore then cement them onto this

Yep, should of put 2 inches of hardcore down then flattened it with a whacker plate before laying patio.

I'd see if you can some how get your money back off him and get somebody in to do it properly.

Has to have a firm base with compacted hardcore otherwise flags/cement moves
and breaks the joint cracks and falls apart.
Tell him to carry out the work again gives tradesmen a bad name.
Good Luck

hi paul
I,v just put a quote in for paving, i would use 10mm scalpings to a depth of 125mm/150mm. lay slabs on a bed of mortar (not spot beded) and use a dry mix for pointing and just brush in to joints.

I beleive i have just offered a quote for this work,but in response to your question due to the weight and size of the paving involved a stone base is necessary to provide a firm foundation.

Just looking at the answers you have recieved here all good advise, you were ripped off by the builder you got in probably because he was the cheapest quote. The amount of times i have quoted for a job but the client has gone with someone much cheaper, but they only end up calling back in a few months asking to repair what the cheap quote builder has done or not done in most cases.

One thing do not just sweep the joints in they must be hosed in properly, it takes longer but means its done properly and lasts longer.

Good luck.

Utter rubbish, your sub base is the most important thing. Its guys like this that give us a bad name.