An hour after HW would be a good time to leave. As Damo said you wont get down in time to go straight in but that would be virtually impossible to do at small yacht speeds no matter when you leave.
If weather is suitable you could anchor in Blue anchor bay a mile or so West of Watchet. Another possibillity is to drop down to Barry first.Kill an hour or so in the harbour then head across .The tide will then be more or less on your beam from Barry to Watchet.
No need to wait until nearly HW to go in It pays not to go in immediately the flap opens.Leave it ten or fifteen minutes when the currents die down.Go through the marina entrance fairly quickly to minimize the effect of the cross tide on the entrance.Rig lines and fenders in Watchet outer harbour which you can enter about 20 minutes earlier than the Marina opens.
We will probably use one of the same options coming down from Cardiff. We can do it on one tide but prefer to take our time and enjoy the sail rather than be constantly looking at the clock to make sure we get there on time.
Fingers crossed for suitable weather look forward to seeing you there. Cheers Graham