Timothy Murphy wrote:
Can I get a spray which will seal the wall?
Or should I apply something with a brush?
And if so what?

Tim,
I have sealed porus pots and a toilet bowl, with an invisible
hairline crack, using PVA glue diluted to a milky consistency. With
the pot I just swilled it around for a few minutes, left it to stand
for ten minutes and then threw the excess away. With the toilet I just
left it in the bowl overnight and flushed it away in the morning. That
was 24 years ago and we're still sitting on the same one.

If your wall isn't too big you could apply it with a plant spreyer.

Chris.
PS - (Played Cricket on the grounds of Trinity when it celebrated 400
years.)