The pump is working, the reservoir is full.

However, when activated, the wipers start their sweep then fluid squirts out of the nearside jets, followed a second or so later by fluid squirting out of the offside jets.
Any ideas what's going on?

I've had the tubes off and blown through them, there's nothing blocking the tubes or the jets - but why do the nearside jets work first when they are furthest from the pump? The fluid has to pass the o/s jets.

It's annoying as the screen gets dirty very quickly with road spray & the wipers do a dry sweep before the washer fluid is squirted out.

The antisyphon valve in the 'Y' piece moving ok?

Cheers, I'll check it out.