Every now & then I get a free weekday & wondered about just turning up at the BMX track & having a (pathetic) go at some jumps

I would be on a mtb, not BMX, don't want to race & haven't time to join a club

(generally or specific to Bournemouth) Is it likely:
Acceptable just to turn up & have a go
A good place for a near beginner to start jumping

As far as I'm aware, you can just turn up at Bournemouth and ride the track (a couple of my team are from Bournemouth – both national No.1s this year – Jon Moore and Jordan Hatcher).

as mike said

ACE, cheers you two
Mike, maybe you should warn them off for a month or so – risk of low-flying projectiles

Is it a good track (for me – mtb, little skill) would you say ?

you'll be fine.