So ive seen your 2nd comment, how did the water leak show CO leaking?
In terms of loosing pressure a flush will do fuck all, so that engineer was a clown. You need to find the source of the leak first, it could be a rad valve but if your having to top it up that much it'll not be that.
There is a pressure vessel in the boiler, they tend to go flat and should be charged on a service, they do burst tho and that could cause it or there could be a leak in the Heat exchanger and its leaking into the condensate so you'd never find it. Could also be a leak under the floor too. If you have over pressurised it and the PRV has opened they close but usually never fully seat again and that could be another point of the leak.
We dont just go out and suggest new boilers willy nilly but depending the age sometimes your throwing good money after bad money and it can work to your advantage changing it.
Boiler age your looking at around 10-15 years for a boiler these days they certainly aren't built to last 20/30 years thats for sure. 15 if your doing really well out a combi.
People will change there car 5x over than change a boiler even though you probably use it more than a car.