Confirmed, fans off Fiesta's fit. The one already on my car had the Ford Part No 93FB-8K620-AA still written on it, so quick eBay search found a match and I await delivery.It's apparently from a Fiesta, but I have no idea what version
Blank cap turned up today, so I'll be waiting until fan turns up, then off with the tank for a flush, on with the new fan and complete checks to ensure all working.
Been thinking through the cap situation. I agree makes no sense to have 2 pressure relief caps. That's definitely wrong. From trying to rod out the connecting pipe I know the pipe is always open to water flow to the expansion tank side, irrespective of the state of the relief cap (if fitted to front tank). Hence it shouldn't matter which cap is blank and which is a relief valve. They are both effectively closing the same container of water and air. The only logic in having the relief on the rear larger diameter, is that it will help keep the connecting pipe from blocking. Thing is, there needs to be an overflow pipe on the cap that is relief cap and on these tanks, that's the rear, larger cap. If front cap is the relief, water will just leak out the top of the cap! And drip down tank and wing onto the exhaust.
Tony