is it possible to obtain an adapter to fit 3"" plastic downpipe to "old" caste iron guttering?

Yes they do adaptor should be able to get from most plumbing suppliers, if no luck then available from bes.co.uk part number 12310 white 12311 black both retail at £2.79+vat Regards KPS Intsallations Ltd

Yes. Take the internal measurement of the cast iron collar, builders merchants or plumbing supplies should have something to fit. If the cast is a small diameter you may have to start with a mini gutter adapter, then work up to the 3in. Failing that, renew it all in plastic, or reclamation yard for some same size cast iron.

Yes, you need a fernco coupling

i had this problem a little while back and used a company called fernco unfortunatly i had to ship it from the united states but it worked really well it was just a rubber connection really hope this helps

tis possible ,but a lot more beneficial to rip out and replace stack in plastic