Ford KA juddering when cooling fan comes on.

On a car that age it should still be under warranty. Take it back to the dealers.

Yes, I'd thought of that, they want to charge the earth just to have a look!

You have vastly cut the odds on getting it repaired under warranty by removing and testing yourself .Any such moves are grounds for rejecting warranty repairs if dealership sees signs of this .All depends on dealership .

Warranty also dependant on mileage and car have been serviced as per Ford schedule .

Yes, I understand all that but has anybody got any idea what's causing the problem?

You sure the juddering is a form of engine miss fire?
Do the revs drop to below idle speed?

The idle speed stays the same and there's no misfire when the fan comes on but the whole car judders.