Ladbrokes thinks that the most likely outcome is Labour getting between 40% and 50% of the vote, offering 5/4 (44.4%). The next most likely outcome is between 50% and 60% of the vote, with 7/4 (36%) available. It’s also offering 5/1 (16.7%) on Labour getting between 30% and 40%. Those who are very optimistic about Labour’s chances can get 10/1 (9.1%) on more than 60%; those who think Labour will do unexpectedly badly can take a punt on it getting less than 30% of the vote at 16/1 (5.8%).
My guess is Labour will win with a reduced share of the vote. In three out of the past four general elections Labour’s percentage share of the vote in Newport West has been in the 40s. Labour just managed to get above 50% at the last election, but its polling ratings have slipped since then.
A delayed Brexit will probably see some protest votes going to Ukip’s Neil Hamilton; a successful vote for PM Theresa May’s withdrawal agreement will do the same for the Lib Dems. Although this won’t be nearly enough to change the outcome, I’d suggest you take the 5/4 (44.4%) on Labour getting between 40% and 50%.