Absolutely not.
We dont increase or installation quotes on a seasonal basis. Yes its true we are busier in winter with breakdowns heating not working and so on.
Installs are fairly steady, most of us will be doing two or three a month with breakdown and services in between, not to mention plumbing work leaks, installs and repairs.
I was reading your thread last night, and before Dave had posted a guesstimate figure, I was thinking £6500+. and Im in the northwest, Dave is Norfolk where prices are dearer, so gave you a very good estimate.