Which valve are we talking about...
There are two likely candidates: The expansion chamber valve and the overpressure release valve. Since the overpressure release valve ought to be piped outside in 15mm copper pipe, I expect that we are talking about the former.
If its the charging valve that's leaking (i.e. the one that looks like a car tyre inflation valve), then that is a cause for concern since there should not be water in that side of the chamber at all (the wet side connected to the heating should be separated from the air side by a rubber membrane).
Assuming the expansion vessel is shagged, then you really have two options. Replace it or augment it with another somewhere else. Replacement difficulty varies with the boiler - some place the thing right at the back such that the only way to get at it is to dismount the boiler from the wall. This is obviously a non trivial job since it means breaking and re-making all the connections for gas and water etc. Others have them in more serviceable places. If yours is of the latter type, then you could investigate getting the makers replacement part[2], swapping it out and pumping up the new one.
If you can't easily get the old one out, can you stop its valve weeping (by sticking a cap on it for example (perhaps with some Fernox LS-X leak sealer in it to make sure it does not leak!)? If you can, then you can get another generic expansion chamber[1] and add that somewhere else in the system. (you can leave one rad with some air in it as a temporary measure assuming you can stop the leak).
[1] e.g.