I think the Lib Dem's will win it with about 18,000 and the Conservatives will drop to around 15,000, Labour will come in 3rd and the Brexit Party will come in 4th, with the MRLP hopefully giving UKIP a run for their money for 5th.
Here's why:
1. Reselecting Chris Davies was a mistake after the scandals he's faced. The "Boris Bounce" may be less significant where constituents are sceptical of the individual candidate.
2. The "Remain Alliance" - very smart move from Plaid/Greens not to run to bolster LD support. None of the other parties are explicitly a remain party.
3. The left-wing leave vote will be split between Labour, the Conservatives and the Brexit Party (less so, but think of Claire Fox, for example)
However, I do foresee consequences in the future for the "Remain alliance" in a General Election, directly as a result of this by-election. There would be furore in Scotland amongst the pro-UK, pro-EU vote if the Lib Dems didn't contest a seat in favour of the SNP, solely to avert a no-deal Brexit. Economically, orange-bookers and the Greens are quite substantially different. I'm not sure how comfortable many economic liberals would be, voting Green just to avoid a no-deal Brexit.
Does anyone have a more detailed expected time of the results at all?