[MatthewH]

Thanks Peter. I have purchased a replacement relay for £6.50 including delivery (aftermarket German-made replacement).

Since cleaning the pins on my relay earlier in the week, I've not had the starting problem at all. I think this is just a coincidence as it seems to go in phases - it'll fail to start every morning for weeks and then it'll work fine for a few days.

I'll post an update when I've fitted my replacement - if it all works correctly I'll post the part number of the aftermarket relay I purchased.