Well last night was a LOT better. He woke up crying about 5 times and twice gently bit her neck and put a leg over her back but decided against it - progress! Took them both to the vet today for an introductory look-over and chat. The vet reckons he's very mature for his age but extremely unlikely to be fertile. Thank god it transpires they agree with early fixing at that practice so they're booked in to be done at 4 1/2 months.

Thanks everyone for your advice and support.