This was a fairly simple one.
Original switch was mounted on the rusty metal plate. Water had obviously got under the rubber seal, corroding the original switch (not-shown) and the plate itself.
I eventually found a direct replacement switch but it was expensive - £40 + VAT. Seemed excessive for a push-to-make switch that operates a relay so I used a cheap plastic one from Maplins that just cost a couple of quid. Originally I put this back on the steel plate but despite cleaning it to bright metal, it rapidly corroded again. I could have painted it but I decided to make a direct replacement in ABS. Very easy to model and fits perfectly.
Been fitted for about a year - works great and cheap as chips.