A little advice please - wall prep before split face tiling

[Qwerty]

Thats an extremely thin skim of plaster and then render underneath. Looks a very friable surface. I would scrape back/ rub off high spots, clean, prime, glue & screw (mechanically fix) tile backer boards to the wall.

 

[GAZ5518]

I think the what you are describing is 'sand and cement'.. Which is how tiles were fixed before the introduction of Ready mix and powdered adhesives.
With a bolster and hammer you can get behind it and chop it off( wear gloves)!... Failing that if it's as stuck as you say ... Bond / fill in the gaps and Tile away..( A Tiler would probably fill in and fix at the same time using a flexible powder adhesive)... Good luck .

After seeing the pictures .. I stand corrected...It looks like serrations on the wall over finish plaster..
Tiles look like 4 1/4s pilks...Stuck well and can be a bugger to get off..A few hair line cracks... which if has blown you should chop off and make good
As mentioned already ... Quickest fix is to overboard

 

[giveitago]

Some were murder to get off - a particle at a time! Others just fell off. The whole kitchen and utility room are the same. I've already detiled utility room and still the rest of kitchen to do. I'm still leaning towards backer board for that wall. the other walls are just getting plastered as normal. The SDS remover made all the difference , although the high up tiles seemed fixed better so a bit of a strain.