I am about to replace a tiled pitched valley which has been leaking on my
50's semi (facing west in the west country).
I have got a pamphlet from British Lead Mills giving details and went
to a merchants who advised fitting grey GRPF sheeting like most new
houses.

Could someone advise on the following:

1. How weather resistant is the sheeting?

2. The pamphlet shows a piece of 25mm beading (square cut at 45 degrees)
running the whole length of each side of the valley to prevent water
running up the sides. Is this really necessary or is it overkill (two
roofers told me they did not do this)?

3. the width of lead specified for a 150mm opening with bedded tile is
given as 550mm, is this OK.

4. The pamphlet says it should be made with grade 5 or 6 but I have been
told grade 4 will be OK, any advice here!?

Thanks