Hi Michael
Lots of advice & comment on this but if you will take a last tip from your uncle Chris, get somebody in to change the rad valve as it is leaking & the system pressure will drop again, over a short time. At the same time get them to do a service on the Megaflo un-vent cylinder, these need to be looked at from time to time (manufacturers say once a year just like boilers) as they can be potentially dangerous if things go wrong. I am not trying to scare you & the photo is not that clear but it does look as though it does not comply with Building Regs G3 & if it has not been looked at in a while it would be a good idea to do the two things at once.
Uncle Chris