If all you are doing is re-pointing, re-rendering and swapping out a few bricks then there's no need to comply with the current Building Regs regarding the height of the railing.
I would echo that, as works are being done, that increasing the height of the railing would be prudent. If there is currently just a wall that could be topped with a handrail system - such as Key Clamp - very easily and for not a lot of money.
There is always scope for misinterpretation.
If my posts can be interpreted in two ways, one that makes you feel angry and one that doesn't, I meant the latter.
Everyday is an opportunity to learn something new.