All worked out well in the end thanks JamieABZ!
So we got to Gatwick, went to flight connections, they had booked us on the EI from Gatwick, but this didn't suit for various reasons (mainly didn't want to take lots of checked luggage to DUB and was getting picked up from LGW to drop the bags home before flying out from LHR)
Flight connections changed the ticket onto the late LCY-DUB which was very nice of her. Currently at the Crowne Plaza Dublin Airport, fly back to Heathrow at Lunchtime.

The thought of eu261 crossed my mind but I think it would be a tough one to prove.
The first two hours of delay were maintenance issues so do qualify.
The argument about the remaining 3:17 would be how much of this was passenger action (I view as extraordinary and so not qualifying) and how much was within BA's control (the baggage offloading and reloading was very slow and there were further delays whilst the flight plan was altered). I'm also of the opinion that we probably went into "discretionary" time and so am greatful that we did get back at all, although I did joke that if we were bussed to The Orleans (as happened post the engine Incident) it would probably be late enough for the after midnight $1 ten pin bowling there!
Vegas was great thanks, won enough to pay for the hotel and enough shopping to use the whole customs allowance. When are you in Malta?