Hi we have also had our flight moved from 9.25pm on a Friday, to 8am on a Monday with Portland going to Dalaman.
Ours was on 21st July, and we could no longer travel on the flight they changed us to, so we had to change holiday dates to 24th July.

Our original flight was also from Gatwick too, so not as bad as you.

What reason did they give??

no reason yet, just a standard letter

I'd be really annoyed with something like that, it seems like a major change to a booking moving the airport you depart from, for example you might have already booked and paid for airport parking.

I thought Portland were supposed to be quite reliable. (Hope so I have two holidays booked with them for this year ) I suppose it must be something to do with the actual aircraft they use or possibly how many seats they have/haven't sold on the Luton flight. Didn't they give you any details as to why? Seems a bit poor if they didn't.

Wonder if you can get them to give you free parking or something similar as compo?