Blimey, wish I hadn't posted now, I feel like I'm a really bad owner now, reading through those replies!
A bottle has been in with them from day 1 - their mum used a bottle, they just never have. I've had them all out and gently put the bottle to their lips so they all know what it does.
The suggestion to borrow another guinea to show them - no way! I have 5 other guineas - all girls - don't think I'll put them in! And to introduce any males would be stupid - it'll cause fights.
There's no point taking them to the vets - it's all of them, not just one, and they eat everything fine.
I change the water bowl frequently - it is not possible to tip it over and they can't wee/poo in it, wrong angle!
They have fresh food 3 times a day, always cucumber/grass first thing and last thing because I don't want them going too long without moisture.
Thanks for the suggestions but I'll stick with the bowl for the moment.
ps Glynis - 'stupid people never showed them' - maybe Duke & Sully just didn't take to a bottle - like mine.