expansion vessel pressure?

we have an oil fired combi with a separate free standing 24 litre expansion vessel some 3 metres away. Following recent pressure problems we emptied system, checked expansion vessel which seemed ok and recharged it. our boiler is set at 1.5 bar cold. We called manufacturer and local plumber to ask required pressure in expansion tank and were told 1.5 bar (must point out here only spoke to office/shop not technician). Now no problems with loss of pressure in boiler but only rises to 1.7 bar hot (same whether boiler set at 70 or 80 deg. c).
have also read that expansion vessel should be charged to LOWEST operating pressure of boiler which is around 0.75 bar. Should we let some air out and how?

Marymar
January 2012