VisionSet wrote:
I've just redone the flashing seal with the brickwork on my flat bay
window roof. The depth inserted is about 15mm so a bit less than
recommended. I've directed a hose at highish pressure along the joint
for a few minutes, but it still leaks. I've simulated I guess an
attrocious storm but that's not really acceptable I don't think. Can
I treat the mortar I've applied with some silicon treatment to reduce
ingress further? Or what?

TIA,
Mike W

You'v probably washed the mortar out of the joint... I would suggest taking
it all off and starting again, no amount of silicone will ever seal it.
If it's a long piece of lead (more than 3ft long) you will need two people
to get it in to the right depth, (25mm+) use an angle grinder with a diamond
disk in it (new, preferably so you get the deepest possible cut) and wedge
it in with small rolled up lead wedges.
Point with a strong mix 3:1 and hope for dry weather for 24 hours.