Back boiler leaking - seal off/replace?

Just discovered the source of a damp problem close to our fireplace - had a dig behind the plasterboard on chimeny breast and it looks like back boiler is leaking. We have OFCH too so can do without back boiler - on the assumption that this swill be a relatively big job to replace.

I believe though that a back boiler can be fairly easily & cheaply disconnected (by a professional I should add, not me) and that we can still light the fire if needed?

Can anyone advise?

Would you consider a medium size boiler stove to compliment your oil heating, would be a good time to do the upgrade as you must dig into the chimney anyway to complete the repair, I can recommend it as a possible way to go.

Thanks - yes, this has been mentioned to me but I want to keep cost to a minimum so I think this option is out.