Expansion vessel full of water/or knackered.
It is quite easy to drain boiler remove x.p and blow the water out and re-presurise it to 1.5bar and re-fit it.
That sort of money can get your boiler repaired and keep it repaired for a whole year......just in case something else fails...HEATEAM will repair and maintain gladly...
Do a lot of these Johny and i must say removing the xp and pumping out the water is an excelent method i would like to bet it is not fooked. Watch this space
isolate flow and return drain boiler via drain off not prv,remove xp vessel take it out side and attach a foot pump to it and blow that water out,re-presurise and refit.
Are you mad ?
There is a procedure to adopt if you suspect you can "smell gas",
In fact if you look at your gas meter there should be a sticker telling you what to do if you can "smell gas".
Take a look when you are turning off your gas supply.
Seen one with massive icicles hanging from it yesterday,it looked really pretty,my crapy 2mp camera would not do it justice............Must be the weather