So, who do you bank with?

Kim_13

Halifax (payments set up there) and Chase for day to day debit card transactions. 

ZeroSum

What's a 'main' account?

Salary is paid into Halifax
Bills come out of santander
Lloyds for credit card stooze DD's
Nationwide for Mortgage stooze hub
Chase for small spends
Virgin for joint account (glorified savings really)
There's a few others, I don't really use that much.

Volcane

Santander - salary pay in and bill DDs ( some of which pay cashback ) and my longest serving account
First Direct for its link regular saver
Natwest for its linked regular saver
RBS for its linked regular saver
Lloyds for its linked regular saver and Disney+
Halifax for its free fivers
Nationwide for its linked regular saver
Chase for my eBay transactions 
TSB for charity donation DDs
Starling… currently no use and only has a fiver in it, will be used for next switch
Barclays… very recent switch, undecided if will keep or switch out 
Yorkshire BS for its debit card accepting savings account