So, what would be a reasonable price to have in my head when
thinking about it? £1.5k or £3k a bay? I realise things can
vary a lot between companies and areas, but I'm just after
a ballpark figure.

What material? Sash or casement? How many opening lights?

Supply only, or supply and fit?

AFAICT, the price fitted will be about 10 times the cost of materials,
unless decent hardwood is used.

Christian.