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Actually I've just noticed on closer inspection that the red bit seems to be "stuck on" rather than a marking.
Could it be eggs?
I haven't got my spider book to hand so if you have no luck with ID,will have a look in my book when I can get it,must say the title made me smile.
Yes, it does indeed seem to be a harvestman.
And I've just zoomed right in on the original high-res version on a better screen - I think I can see eyes on the red lump so clearly something is hitching a ride!
It just fooled me as it was so well aligned with the spider.
Thanks for the assistance, and I'm glad if the title made anyone smile!
Lat/Lng: 50.91266, 0.15084
OS grid ref: TQ512147
Found on the path between beach trees