just to give a slightly humorous sum up of how this went, I checked in my bag all the way through

However, I also had quite a large laptop bag, and had resolved I would go for breakfast at flounge then exit via the c gates train.

I somehow managed to find passport control though before I got to any flight connections after the C to A train.

So I was outside the airside section before I knew it, and was just going to go back through T5 security to get to flounge.

When at the lift to go up I saw Airline lounge floor 1, and it hit me the arrivals lounge ! I went in and asked the guy if I were to leave the airport and come back could I leave the bag, and he said yes no problem, even though a sign I saw later says you cant leave luggage if you leave the lounge.

I also discovered the showers, the ironing of clothes and that you can get a razor off BA , that is some set up those showers, I hadn't used them before and trying to work out how I wouldn't have to prance naked around the lounge was a humorous thought for a fleeting moment before I actually did wander what would happen if they didn't return the suit and shirt

So all worked out I got into London, left the bag at arrivals, and then picked it back up an hour before the main flight and went to Galleries North as hadn't long to wait

Not sure it resolves the original question but thought I would share it ^