I didn't think the model MWC asked for installation method - but maybe some derived ones do...
Anyhow, my policy would be you can do whichever you think most appropriate for the circumstances - so long as you make it clear. The point of the exercise is to communicate with the human who will read the certificate in the future, rather than to blindly follow some arbitrary procedure.
So I'd either pick the "worst" of the two and use that, or just use the one that applied to the new work (which is what the certificate relates to after all) - if necessary making that clear with an extra note.
Things can get messy where installation conditions - and indeed cable sizes - might be different for the new & existing (e.g. 2.5mm² surface run spur from a ring that's been wired in 4mm² for reasons of excess thermal insulation) - or 2.5mm² spur from a 4mm² radial - as the model MWC seems to presume uniform size throughout the circuit. Entering 2.5mm² and method 103 is going to look needlessly dubious, likewise entering 4mm² when you've (correctly) only installed 2.5mm² is similarly inviting censure. So record whatever you think will be clearest - if that means adding more information than the form asks for then that's fine. They're only model forms - not straight-jackets.
