Christian McArdle wrote:
Now, the lower tiling was done over the older tiles so sits quite proud
of the wall so any new tiling above needs to be at the same level.

So, my question is, what's the best way to pack the wall out to the
same level before tiling?

Remove the existing tiles, both layers. Attempting to extend the current
field will look manky. The tiles won't match, either.

Christian.

Ta, for all the advice

I'd like to redo the whole thing but don't have the time.