there is a roof repair material coating on the market that can be applied to lead or felt roofs it contains a fibre coating so it will last a lot longer than just bitumen coating

The only proper and sure way to fix your lead valleys is, strip the tiles out either side, and relead them.
Check your lay boards are ok first, and no rot, lay building paper with new lead valleys, retile back both sides, If your tiles are bedded in cement in your valleys, I tend to fit some asbestos free board, cut to size dependent on overlap up the lead valley with the tiles bedded on this, with glass fibre strands in the mix for strength.
You always get movement with lead, and the mortar cracks, but bedded onto the asbestos free board, the lead can move/contract independently of the mortar joint.
There are glass fibre valleys you can get, which is a lot cheaper to buy and fit, but I prefer lead, dont fit the lead in one length, needs to be about 1.5/2m sections code 5/6

Idealy you,d remove the existing lead and replace with new or GRP fibreglass. As a repair that lasts a while you could try Cromopol. You just paint it on, can even be used in damp conditions.

Remove tiles either side of valley, remove lead, inspect valley boards. Apply a breathable felt over the valley board. Fit code 5 lead in 1.5 m strips nailed one side to allow for expansion. Re fit tiles or slate, bed tiles back on to a/c strips and point up job done

There is no way of replacing valley lead without the removal of the valley cut tiles from both sides. These should be stacked in order and set aside for re fitting. Valley lead is usually 390mm- 600mm depending on the width and a minimum code of 4. Depending on which code you purchase is depending on what lengths the new lead can be cut to. Install the new lead fix on one side only. This is to allow the lead to expand and contract. If it is nailed on both sides it will split again. Re fit the tiles and re fit the ridge. Hope this helps. Its easier to install a valley than write down how to fit one.

My advice would be to Contact a respected Glass fibre company and get a quote. Mds mouldings in Manchester is a good one. ;-) Be cautious of letting just anyone tackle it make sure you see some previous work. Theres a lot of damage to be done with an ill repaired water channel.

hi,the best option is to get a roofer around and give the area a overhaul .and he can see the problem has it is high up.get someone who is a roofer with good feedback.many thanks kentishbuilder

Renew in lead but is very expensive
renew in glass fiber just as good too
Or quick fix reline useing torch on felt, this option want last as long and is realy only a quick fix

The best solution to carry out a repair as per your request without replacement shall be that the valley and abutment tiles be scraped and wire brush prepared and a liberal coating of a fibreglass waterproofing solution be applied to the valley lining and to an approximate 20mm or so lip onto the abutment tiles .
This all form a good string watertight seal for a good period of time but needs to be carried out with care and attention.