While I share the OPs risk aversion, both BA 11 and 15 depart in the evening so if they left Manchester in the morning, either by plane or train, it would still leave plenty of time to get to LHR in case something did go wrong.

A 4-6 hour buffer on a separate connecting flight should be enough. I did something very similar travelling to and from SIN this summer as I realised there’s more than enough time in the day to get to LHR without the need to overnight in London.

But if not, I agree with the other posters in recommending an airport hotel. Get a late flight from MAN, have a relaxing morning and then head over to the CCR for the afternoon.