i intend removing them and connecting into the 22mm pipes. I think it
will be best to fit an expansion vessel at the same time, just a couple
of questions, can the vessel be fitted anywhere on the system,? can you
recomend any particular make? would i create another problem by
oversizing an expansion vessel?

Some boiler manufacturers state locations for the expansion vessel. However,
as this is an additional one anyway, you can put it anywhere on the system
that can't be valved off from the boiler. You can't (within reason) oversize
an expansion vessel. I would probably calculate for a 0.5 bar rise, which
gives loads of headroom.

Christian.