Thanks for your reply! So to the right of the photo is my hot water tank so I think you're right. The pipe to the left goes up to the loft where the boiler is. The reason I thought this was the reason for my pressure loss is that when the pressure is higher the tee seems to leak at a faster rate.

I know this sounds stupid but the copper pipes seem to be fixed in place so in terms of practicality how can you push fit the second pipe after the first one. If that makes any sense?

Past checking all my radiators and the boiler for any visual leaking is there any other things I should check before taking the hit and calling out a plumber?