Taking the plaster off and replastering is the best way, but obviously could get very expensive.
The dot and dab plasterboard then skim method is a cheaper way of doing this, but would mean door linings would probably have to be replaced for wider ones, etc.
Certainly don't want to push you into extra work not allowed for, though dodgy old blown plaster can be a nightmare...
Got a builder coming for the second time tomorrow with his plasterer and joiner. He has been recommended and he said I should go and look at an extension he built (which I did). Had a word with the guy whose extension it was and he said he had no issues etc. I will ask him. Fingers crossed.