So, just to conclude this, I siliconed everything and was suicidal when it was still squeaking. When I could I emptied the boot of everything and discovered the spare wheel wasn't quite clamped completely solidly, a few turns to tighten it has completely resolved the issue... getting silicon spray on North Uist was a challenge and cost me £8 😂 - oh well, sorted. Thanks all for your suggestions 👍👍👍