You're right!
I could see that both valves did come on together, but the water
still kept coming only from the left.
I've prodded a bit of plastic sleeving into the relevant holes, and
this seems to have freed it up somewhat. The water now comes from the
left and the centre
The crap thing about this particular machine is that you can dismantle
everything except the plastic water/detergent dispensing thing.
Thanks for your help.

To get the dispenser out on most WM's pull the dispenser out fully and
lift
it upwards slightly then a sharp yank towards you will get it out.
--
Sir Benjamin Middlethwaite


That's the dispenser drawer, the dispenser is the top of the box the drawer
slides into.