When fitting these unions to new boilers I normally use PTFE tape which does the job.
If your not happy about that you could use Loctite pipe sealer, I think its in the Screwfix Cat.
If you know the tray you are going to buy go on their site and obtain installation instructions.
If you have a basin in same room that waste will be plumbed to an outside drain the waste from your shower can go into that.
If you want your tray on the floor you will have to chop out the...
Sounds as if your system needs flushing.
Use a propietry Sludge remover and follow instructions on container.
I would use a couple of litres
The longer you can leave it in the better it will work.
Dont use a cleaner containing any acids.
Recommended you have same capacity of stored water in header tank as cylinder but as you are only a few litres out I wouldn't worry unless your demands for hot water are excessive.
You could join to header tanks together with 22mm pipe and tank connectors in line.
Make sure the new header...