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Thanks for your responses everyone - I thought it must be a sign of trust & affection but wanted to check. I've really enjoyed all your stories, SlackAlice's was very touching.
Yes I've seen him quite recently, most days in fact. My partner was so taken with my descriptions of him, that even on Saturdays we take a slight detour so she can see him too. One time, we were both crouched there stroking him, when he suddenly turned and walked purposefully over to my partner, looking right in her eyes, then reared up and rested his paws on her knees, sniffing up towards her face and looking her right in the eyes the whole time, then tentatively tapping his paw on her chest - almost as though he was checking her out, wanting to be friends as she was with his established friend (me).
He doesn't have a name tag but I've named him Arnold, as it somehow seems to suit him. Nearby, I also made friends with a female silver tabby who's very affectionate - she even once jumped up on my leg as I crouched down to stroke her, rested her paws on my shoulder and sniffed all over my face, purring. There are a lot of cats there - I twice saw a very unusual-looking one, almost milk-chocolate coloured (bordering on Caramac!), quite friendly, with a name tag that amusingly said 'Hampton'.