Replacement of lead roof valley

There a damp patch in the landing ceiling of my victorian terrace, so I got a quote from ratedpeople. The guy who came around was highly rated on the site and seemed very knowledgable, but flagged up a number of other issues with the roof that needed sorting out. As we knew that the chimneys needed attention from our purchase survey we agreed to have it all done, at this was essentially the work we had been putting off since moving in plus some necessary maintenance. This came out at £650, including reflashing the main chimney, repointing both chimneys, resetting the ridge tiles, misc other small fixes and clearing out any debris in the gutters. That seemed vaguelly reasonable so we went ahead.

However, after actually seeing the roof, he has decided that the actual leak problem is an issue with the rear lead valley. It's the original, so over 100 years old, and has been patched up several times, apparently with little success. For this he has quoted an additional £750 to replace the lead valley, which will obviously more than double the original quote. Obviously lead is expensive and there seems like a lot of work involved, but from Googling I can't get a handle on whether this is a fair price or not. Old forum posts seem to suggest lower prices can be had, including scaffolding, which is this case will apparently not be necessary, saving further expense.

While getting all the issues with the roof at once seems sensible, we do feel a little railroaded into making a snap decision on this, so if anyone has any thoughts on this we'd be glad to hear.