Thanks for the quick reply.
So it was LHR-> ZAG 23/10. Returning ZAG -> LHR on 31/10
There's been about 4 time changes on the booking so far but then today they cancelled the return flight and moved it to 30/10
At the time, we booked that flight as it was the best price at the time. Looking on the BA website now, there's a flight
LHR -> DBV 23/10 Returning DBV -> LHR 31/10
That would be perfectly fine for us and allows us to keep to our original dates and fits in with accomodation stays. It's whether the distance is too great and whether they would allow us to change it at no penalty as current prices for that flight are expensive + some of our seats were booked on Avios.