So over the weekend I had a chance to take a closer look, and as suggested cleaned the fuse port out fairly well.. Which made no difference. (although some of the contacts were very dirty)

After looking at various relays, (the four in the sealed box above the fuse compartment) I noticed one was slightly ajar, pushed it back in and presto!

Thanks for the advice guys, it is always appreciated.

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1971 280SE - The S Class
1974 280CE - Sterling
1988 300E - Stanley