Installing log burner and the Party Wall Act

I'm no expert, but I'd be amazed if you needed to serve a notice. This wouldn't be a structural alteration.

The Party Wall etc. Act would only apply if you were either cutting in to a party wall s2(2)(f) or cutting a chimney breast away from the party wall s2(2)(g). The latter does not apply in your case. There is no consensus between party wall surveyors as to whether a chimney breast should be considered as 'party' given it's physical attachment to the wall. One view is that s2(2)(g) identifies it as a separate entity to the party wall, given it can be cut away from it, so s2(2)(f) does not apply. Other surveyors will disagree.

There is however another aspect which is that there are works which are considered 'de minimis non curat lex' which means 'the law does not concern itself with trifles'. If you look at Section 6 of the Government's explanatory booklet (available online) you will see it lists things which it says are normally too minor to warrant serving notice such as hacking off plaster/replastering, drilling a wall for plus and screws or chasing out for electric wiring and socket - those are considered 'de minimis'.

Although the Government are careful to point out that what they say is not the law but advisory, they do state in respect of 'de minimis' works that "the key point is whether your planned work might have any possible consequences for the structural strength and support functions of the party wall as a whole, or cause damage to the Adjoining Owner's side of the wall". It could be argued that opening up the fireplace "a bit more", if done with care, wouldn't be expected to have any adverse affect on the party. wall.