Plan to go down more than the 75mm required especially if the removal method you choose is likely to leave a rough bumpy bottom as SDSing would do. If you were able to side-cut it with an angle grinder, however (tho' I understand this is unlikely...), then you'd stick close to the 75mm you are after 'cos you'd know it would be a neat, accurate cut - perhaps 80-odd mm. With the core drill method, there will still be some 'bumpiness' as the cut cores will still need SDS chiselling out, so I guess 90-odd mm makes sense? Summat like that Good luck - take it steadily - lots of breaks if needed - and please post back on your progress
So..... Job done. To be honest it wasn't as bad as I thought considering the awkwardness of the job and the fact I was cutting into dense concrete block. It took be the best part of 4hrs to take 85mm from the top of just over 6 metres of concrete block. ( two doors and a bifold). I also added a layer of cement, DMP sheet then 75mm insulation. If anyone else needs to do a similar job in the future here is what I thought worked best. I first tried using a core drill but this was taking a long time so I opted to drill lots of holes ( 12mm bril bit). I marked 85mm depth on the drill and went for it. Not sure my drill appreciated half hour of full on drilling as it was very hot when I finished. Next step was to attached a chisel on the drill and knocked it into hammer mode . Starting off with a small chisel then finish off with a wider one to level off. Brushed up the brick work and cleaned down. I then added a mixture of sand/cement to the bottom of the cavity to level off. Cut and fitted a layer of visqueen DPM sheet on top of the sand cement then added the insulation on top of that. I then rested some bricks on top of the whole lot until the cement has dried. NEXT JOB...... I will add a 25mm insulation around the edge to stop the cold bridge between the door and floor, prime the floor then pour on Self Levelling Compound over the whole lot.