Well I did it, no it didnt go to plan, I decided to replace all of it for reasons below. It all went well to start with. I had everything in place and cooked within an hour, but it leaked.
This is how it played out from start to finish. I decided to replace all of it after not being able to sweat the original pipes apart, also after cutting it all out I found the Tee peice was full of solder which looked old so nothing to do with me trying to sweat the pipes.
I think I forgot to put flux on one of the joins so I got the leak. It was the first join I had put in place and I suspect I just fitted it to see if it was ok but forgot to take it off and flux it. After a mild panic attack I got myself together and worked out how I was going to sort it, due to my lack of skills my options were very limited, I have done a fair bit of plumbing but I am by no means skilled. I re-cut the 22mm and managed to sweat the join between the new 22mm elbow and Tee reducer, leaving the small connecting 22mm pipe in the Tee (I would have been stuffed if this had not worked).
I then set the smoke alarms off, so took the batteries out. I then knocked the battery onto my large ball of wire wool and didnt notice for a minute or so
I got it all back together, had to cut some more room on the joist so I had some movment, and so far no leaks. But I'm not holding my breath. I'll put a photo up for the ***** and giggles.
Also had to do two trips to Screwfix. First was to get some 22mm joiners since the ones I had would not fit, possibly because they had been rattling around in a tool box for a while and got damaged. Then I had to go back again because my only spare 22mm elbow was naff, the solder on the inside was all squashed up so I couldnt get the pipe in it.
I'd say I'd never do it never again, but it seems to have worked and apart me making a stupid error it all went well and I saved quite a bit of money. I know if had more experience I would probably have been able to sort the leak I created without redoing half the job.
Thanks for all your suggestions, it help me get the job done. BTW, if it starts to leak, I'm calling a plumber.