I had this problem just as my 56 Zetec 1.8 TDCI was coming up to 60,000 miles. I took it to the Fleet workshop for the company I worked for before I retired. After 10 mins fiddling, they were able to get it shut perfectly, but advised taking it to the Ford dealer and registering the fault. I did and was without the car for 2 days only to be told that they couldn't replicate the fault but would put a note in my records. 3 months later it came back. I poked the catch with a screwdriver, removed the long plastic strip covering up access at the front of the enging, and tried to use a mirror to see how the cable attached to the bonnet catch. There is nothing useful to see. I wiggled the cable and the problem went away. The next time it struck was the most most inconvenient possible time imaginable - but I beat it. I was about to go to the continent for a week so thought that it would be good to top up my washer. Then the bonnet would not shut, so I tried wiggling the outer of the cable - no hope. I had less than an hour left before I had to set off for the boat so hadn't got 3 or 4 days to stay at home while I got a slot with the local dealer to diagnose the fault then wait a day or so for the part and then hope that there was a slot available to have it fitted. Once I was about an hour too late for the sailing I lost patience, got out my small socket set, and removed the catch. The cable had somehow caught where it enteded the catch and just by unscrewing the thing it all freed up instantly. There was no sign of corrosion. I just re-assembled it and it has been perfect. Having developed the 'crazy wipers/crazy alarm fault' since then, the bonnet has been opened and closed around 100 times with no problem so it has had a good test. It seems that the fault is neither in the cable nor the catch, but in how they fit together. This is likely to be because the tolerances on the dimensions are pretty lax. Replacing either will achieve just the same result (loosen up the caught cable and move the catch around a little so that it all aligns better and doesn't sieze up again) but you will have to pay for a part you don't need. Once I had decided to dive in, it took no more than 20 minutes to take off the plastic cover, release the screws holding the catch and put it all back together just like new, but working perfectly!