Thanks for the replies folks

The bricks appear to be good quality, and in good condition on the
whole. I was hit by the idea of not cleaning up the whole of each brick
because only some surfaces will be visible. Obvious once it's pointed
out!

Will be trying out a powered chisel to see if the bricks stand up to
it. I'm going to rebuild a fraction shorter than previously, but two
courses lower as well, so it won't be critical if I only have a few
losses.

Also, the lower courses were based on engineering bricks, so I will be
re-using those for sure.

Thanks again

JIP