OP, you have 2 choices.
1) As many say, change the boiler. But, do not expect new super duper mega efficient boiler to match 30, 15 or even 7.5 years of flawless operation your present boiler lump has given you. If your new boiler does not break down within the first two years, you should go to the church and thank the lord that you have been graced with a worthy appliance.
When the boiler does break down, you then have the privilege to call in someone whose skill level may be two extremes. If you are again grace with lady luck, you will get an engineer who will fix your shiny white lemon for it to start 'steaming' away at 95% efficiency. Here comes the sting in the tail- the bill. What you will have saved 95% less the lowly efficiency% figure of ideal E type workhorse will unlikely cover the cost of parts let alone the labour charge. Nor will it save the planet as your new boiler will often need parts that the makers are hellbent on making a killing (it is not uncommon for a £25.00 fan to be supplied to the heating engineer for £200 or more), and many visits from your friendly engineer.
2) Get the boiler fixed. Upgrade controls (fit TRVs and programmable room thermostat). Upgrade cylinder. Zone the house. Insulate house. Get to know boilers from forums such as this. This will give you time to walk to the finishing line that will ultimately be the boiler change when present cannot be fixed as many parts are now obsolete. You are lucky in this instance with the gas valve.
If you do decide to change the boiler, do not look at the final figure to see who is the cheapest, but ask for quotes or estimates with installers supplying the same make and model and same specification of work carried out. System cleaning is very important as is correct use of materials.
