Cavity Trays

I have a problem with water leaking onto the internal plaster ceiling in a bay window.
It's a semi detached house and the bay is a ground floor bay with a flat leaded roof.
The wall above the bay is stucco rendered and has some wooden beams inset into the stucco.
All lead flashing is intact and every joint between beams and stucco has been sealed.
The problem only occurs when there is driving rain directly towards the window, so it seems the rain is pushing through the wall and running down the cavity.
So, I want to have cavity trays fitted but the standard 2 brick width type E trays are not suitable so I basically want to fit a flat plastic sheet type overlapping trays with clip on ends.
I've googled but can only find the preeformed type E trays available.
Can anyone help with a supplier of flat sheet cavity trays?
Thank you.

You needn't use proprietary trays, as they can be formed from broad rolls (450mm, 600mm) of dpc.

if theres a window above the bay you should check that as well.

noseall said:

You needn't use proprietary trays, as they can be formed from broad rolls (450mm, 600mm) of dpc.

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Right thanks.
Would this need to be pointed into the inner cavity wall or would it be rigid enough to self-support?
Also what would be used for end stops?
Thank you.

alastairreid said:

if theres a window above the bay you should check that as well.

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Yes there is a large flat window above the bay.
It's about a foot narrower than the bay on each side and I have checked that it is properly sealed with silicone all round.
Is there anything else I should check here?
Thank you.