If the knocking noise only happens when driving with the steering on full lock, then it sounds more like the outer CV joint failing.
If it knocks just by pushing the suspension up and down, then it's more likely to be suspension or wheel bearings. If the wheel 'judders' or moves about noticeably then I'd worry... Could well be the suspension arm about to fail. Check the metal near the ball joint for cracks, as it has been known for the ball joint to shear off from the arm completely.
Check also the wheel by jacking up the car, grabbing it at the 12 and 6 o'clock position and seeing of there is any play or knocking. also try at 9 and 3 o'clock postion. If it knocks when you do this, then you have too much play in your wheel bearings. You might be able to adjust them, or at worst you'll need to have them changed.
£90 each for a suspension arm sounds rather expensive, even if that includes fitting. That's £180 to have two done!
Shop4Parts (as advertised by Fiat Forum!) were doing them for about £15 each on a special offer recently. With a decent socket set, manual, and tools it is quite within the realms of the average Uno owner to change them at home.
It IS possible to change the balljoint on it's own, as it is mearly pressed into the arm by means of an interference fit. Ball joints can be brought seperately from motor parts factors (not Fiat, they don't do them), and you have to have the old one pressed out. But I'd recommend changing the whole arm, as once the balljoint goes it's more likely that the arm is rusting through anyway.
Either way, I'd strongly recommend getting your Uno checked out ASAP and to NOT drive it until the cause of the knocking can be found. You certainly don't want to drive a long at speed and have the balljoint break off from the suspension arm.
Oh, it's also worth checking the obvious. Are the wheel nuts done up tightly? Loose wheel nuts will cause knocking noises like you describe. I'd check these first before rebuilding the front suspension!
Hope you get it fixed, and by the way, welcome to the forum!