UFH needs expansion vessel

I installed this UFH system 15 years ago. Sorry about the embarrassing pipework. But it's worked all this time, although it has a leak somewhere so has to be re-pressurised every month.
I want to fix this, so I'm looking for obvious leaks, but I also want to do any other work to keep it in good working order for the next few years...
But having looked at this system anew, I'm concerned that there's no expansion vessel on the UFH circuit. It's obviously not essential because it has worked all this time, but I'm thinking that it might be a good idea to add one, even at this late stage. Or will the plastic and rubber pipework take up the expansion when the system heats up?

It's got a heat exchanger to heat the UFH side - can just see a bit of it top left in the photo. The building is a cottage and UFH is in only four (smallish) rooms, so there's not a massive piperun.

Comments? Should I add in a - say - 5L expansion vessel?