Many thanks for all your help in getting the scan on the website. I'll sort it out a better way next time.
The top part of the drawing ( where the kitchen is) is the new extension and as noted by yourselves the bottom half of the drawing ( between the 2 pillars and back) is the older 1960s part of the house where the kitchen and lounge have been knocked through together.
All the old floors are sound concrete and the old kitchen part ( bottom left of drawing has been latex screeded as it was about 5mm lower than the lounge when the wall was removed. Hope this makes sense !!
The new screed floor in the extension is the same height and level with the old build.
Slight panic, due to my somewhat problems with my earlier 5m attempt, I have had some quotes in to do the 50 metre l shaped area, None of them have mentioned an expansion joint !! Now I am worried cos these are pro tilers ?? Help......