Expansion tank problem

[Chrissie]

Our ancient boiler has had various bits replaced as we go along but when it works, it works brilliantly.  We now seem to have a perished membrane in the expansion tank (chauffagiste's diagnosis).  Said expansion tank is in a very difficult place to reach and he doesn't want to replace it (and is clearly very busy at this time of year anyway...)  We have discussed replacing the whole caboodle this summer with a joint fioul condensation boiler and solar system, while the various grants are in place, but he hasn't had time to do a real devis yet.  This is clearly going to be pricey and we want to get it right.

We seem as a result to have to keep topping up the water level in the boiler, using a little tap on an inlet pipe, otherwise the pressure meter drops dramatically and the boiler gives off horrendous booming and banging noises.  The amount of water in question is very small, (?a teacup full each morning?) but finely balanced.  If we overdo things, when hot the boiler pressure rises to alarming levels and we have to drain water off through one of the radiators!  We are always here, and so can go through this rigmarole on a daily basis, but

a) is this dangerous?

b) where does the air go when we fill the tank up - the little air-release valve at the top of the boiler is apparently no longer functioning either.

c) there is a steam release valve which does work, but he tells me the air won't come out through this, only steam under high pressure.....

It is clear that we cannot go on with the current system, but do we have to rush into a new one immediately?

Views please!!!!