I'm not sure if DFDS on the Newhaven - Dieppe route would suit you. Roger used it for his return this year and we planned to but have re-scheduled it for next year.
Availability showing you would probably need to shuffle dates but it would be more like £300 return. Add in that if you phone your booking, as an over 60 you can access a discount of 10, might be 20% that is not available for on-line bookings. I'm also guessing their screen would be more user friendly regarding shuffling options around on dates and crossing times.
Just checked C&MC site:-
Eurotunnel = £553
Iris Ferries, Dover - Calais = £367
DFDS, Dover - Calais = £432
DFDS - Newhaven - Dieppe = £338
Colin