Sorry this is really long, but need to explain everything.
Seat 1,2,3 irrelevant
Seat 5 VTight Reg £900
Seat 6 Massive regfish ~£2500
Seat 7 Mitch £1700
Seat 8 Hero is playing ~£1800
Game was playing pretty small, with Mitch and I playing most of the pots, then Regfish turned up and its getting much more heated.
Hero is UTG with

opens to £15, VTight Reg 3bets BTN £45, Regfish and Mitch cold call from the blinds. Hero 4bets to £175 (4bet folding vs the tight 3bettor fwiw), 3bettor folds, fish in the blinds cold call again. lol live poker.
Pot: £570

checks round

checks round

Regfish bets £300, Mitch makes it £670, Hero?
Given the way I've decided to play this postflop, I've put myself in this tought spot. Mitch and Regfish have played a lot with each other, and something Mitch told me about him and a couple of other reads led me to be 90% confident that Mitch had a complete airball here. Something like

So in a sense I've just got to work out what Regfish has. I thought both were plausible but much more likely he was betting TT or 99 or 89ss or something because he thought they were good. Mitch reckons that he always checks these hands in those spots where the pot is massive and tries to get to showdown.
But there are very few better hands that Regfish can be betting, I would've thought he'd bet the turn with a J and maybe 44 or 88 (I don't think he peels pre with 44 or 33 though maybe). So it's 88 or a J that he's decided to play really slow.
WWYD? I know the standard line would've been to bet the earlier streets but I don't want to dwell too much on that. I decided to make my hand look like AK as Mitch knows my range pre has to be very strong given i've 4bet the tightest player on the table. I decided to go for a big street of value on the river and make my hand look as weak as possible and get regfish to payme off, or Mitch to spas.
I don't think it makes a difference to the hand but fwiw I was 100% staked for this game, Mitch knew this, Regfish didn't.