I, on the other hand, would recommend a maximum of 3 hours for a first-timer in the Concorde Room, and that's only if you have someone to keep you company and nothing better to do. GC is worth about half an hour.

It's not very clear from the OP's proposed schedule really how much time she has in the lounge. One of her posts seems to be saying she is flying into LHR from Venice and another that she is flying into LGW and transferring to LHR. In any case, if you've not seen the lounge before it would be nice to pop into it and have a quick look, if only for 20 minutes. Mind you, when I'm travelling with someone and we barely have enough time to look into the lounge they think I'm mad for trying "just to see it".

If you've really got a tight connection then there are enough unpredictables in there that you could miss you flight and have the joy of many many hours in the lounge waiting for the next flight, or you equally you could find you've got a bit more time between flights than you thought.