The felt on my 3 year old pitched roof has failed in several places adjacent
to the leadwork of a valley gutter and the recent heavy rains have allowed
water through to the ceilings

I had the same problem a few years ago - it poured down the interior wall to
the living room with disastrous results. The roof had been built with no
valley protection at all, other than the felt under the tiles, which we all
know gets brittle sooner or later.

The roofer sent by the insurance company fitted a glass fibre valley about a
foot wide, laid on felt with the tiles replaced on top. Insurance covered one
valley, but we paid the roofer to do the other one. No problems since, but I'm
waiting to see how many neighbours eventually have the same problem.