I am replacing - what I think are Giocomini thermostatic valves... with COMAP Westherm ones.. and touch wood... the last ones I have done don't leak... 10 turns of PTFE tape.. (Wound the correct way this time) one of the worst parts is getting the old olive off ... have to change all of the fittings as the Giocomini old nuts are the wrong size (so to speak!)

Have you checked your bypass valve (in a loop which comes from after the pump and links into the boiler return. In old systems this usually has a gate valve in it which you need to replace with an automatic valve).

is this even if I am replacing GIocomini TRVs with COMAP TRVs?

Whenever I have nuts the wrong size I......... Anyway....

If your struggling getting the olives off then I'd highly recommend an olive puller, some plumbers will cut them but I'm too nervous for that and too impatient I'll end up damaging the copper tube.

Have finished the final two radiators... YAY! Thanks to all that have offered help - it has been much needed support... Have a great Christmas....

for what it is worth... and maybe for novices.. and possibly for real plumbers to advise different - this seems to have worked...
1. Turn off central heating and wait for pump to stop
2. Place bungs in Expansion tank - Cold feed pipe first then Expansion Pipe
3. Shut off end of radiator that is not being changed
4. Loosen large nut on end of radiator (Union I think?) and using plastic lunchbox type plastic box... empty radiator contents into a nearby bucket - open rad bleeder to help water run out quicker
5. Screw the Union/Rad fitting back on a little... and then undo the old TRV nut connecting to pipe... (if you don't do this then the pipe twists)
6. Either undo the old TRV (I was cautious about doing this until I had loosened the UNION fitting in case of a lot of water coming out.. and it didnt)
7. Remove old Union and old TRV.....
8. For my sins I wound PTFE tape around the new radiator male fitting 10 times.. and that seems to work... (looking into the end of the male fitting - wind clockwise)
9. The guy at the plumbers merchant told me to not put ptfe tape around the olive fittings on the TRV.... I did on a couple of joints as they leaked a little no matter how tight... (not sure if that is necessary or not)
10. connect back up... remove bungs in expansion tank in reverse order... then BEFORE turning heating on... bleed radiators by first turning on the end not touched... and open rad bleed screw until air has come out...
11. On a couple of radiators I had to also undo the opposite end.. as there was not enough pipe work to work with.... on the male end on refitting I put some ptfe tape..
12. One of the worst things is getting the old style dolly off.. a couple just fell off.. and a couple took about 30 minutes to get off... I have just completed my final two rads in about 1 1/2 hours...

When all is done.. fingers crossed for no leaks and turn the heating back on... for my sins.. I also went around again bleeding...

Bottom line is I am no expert and don't profess to be... but in the absence of a plumber this week have changed 5 old style TRV thermostatic valves) I should add that the tool that the plumbers merchant provide to remove old union fitting is only supplied with about a 5 inch rod... and it needs to be 4-5 times this to get enough leverage to undo the union... a long screwdriver worked for me... I should also add that rust inhibitor will need adding to the system and maybe I should have done that on my last assigment.. but on Christmas Eve and not a plumbers merchant in sight open.. that will have to wait...
if whoever moderates the forum wishes to remove this post as it isn't the way things should be done.. then no problem... I have really appreciated all of the comments given.. next time I may well try to find a plumber a little sooner to save me the hassle.. but job done for about £70... Happy Christmas and New Year to you all!

Now you've finished you can add the inhibitor.