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Thank you all. No news, but I was contacted this morning by two separate people in the same area reporting seeing a big tabby - rolling on the floor in one case, so it's sounds like it was a relaxed cat - so I have been up and flyered all round the houses in that area (a massive estate!) - but there are lots of tree-y areas and industrial units including a food packing place. The social club has put up a poster - I'm trying to get to places where lots of people visit who might see him. If it is him, he's heading in the right direction, even if only by half a mile. This cat was seen about 9am tomorrow so I'll take a drive up again tomorrow morning and do some more calling and food rattling. There were apparently two cats seen together - which is what he did last time when he came to me, sought out a house which was cat-friendly in the hope of food.
It may not be him at all, of course - I met another very nice tabby up there this mornng! - but until I know it's not, we keep hoping.