My jets seem to block intermittently and no amount of probing with a pin clears them out. I guess you can get replacement ones from a dealers. Do they just push out from inside the bonnet?

 

mine do.take them off and give them a blow through from the end that the pipe fits.check the pipe isn't blocked,trapped or kinked.

 

Yes, but new ones will block too. Think about it: where is the crud that's blocking them coming from? It can ONLY come from the pipes/fluid reservoir.

You need to remove the entire system and refurbish it. Clean out the tank and pump with clean water, removing any obvious debris.
The polyhtene pipes are an obvious culprit, especially if you have used cheap fluid, or washing up liquid. I did, years ago, and the inside of the pipes was coated in a substance like mucus

I ended up replacing all the pipes with fresh stuff, cheaply had from a carshop.

Oh, and soak the jets once removed, then BLOW the crud out FROM THE OUTSIDE, ie: in the opposite direction to what it normaly goes

 

Cheers TT - I did replace the pump a while ago and flush the tank as it was full of gunge as you say. Hadn't thought of changing the tubing though - good idea and an easy fix.

 

I'd RE-flush the tank and pump too