your expansion vessel need pressurised, that'll account for the pressure problems. you also need a new domestic hot water diaphragm, you can get a full service kit with all the washers & both the diaphrams for about £35, and another for around £30 to service the 3 way valve. Sounds like you may also need a boiler thermostat, another £25 or so and a new safety valve £10. Would take around and 60 - 90 mins to replace all these parts so maybe a couple of hundered quid to sort it.
these are problematic boilers now due to their age, and some parts are obsolete, so it may be best to put the money towards a new one.
Ian
October 2009