Thanks, I'll bear that in mind should the need arise again.

The whole process was not easy as at first the person I spoke to thought it was their computer and tried to get a colleague to do it on theirs. When that failed a supervisor was consulted and the requestvwas priced as two changes. When I queried the second charge and the telephone fee, it was referred up to someone else who offered to waive the £30 telephone charge but not the second £150 change fee. After 25 minutes I was losing the will to live.
Then when I went to do it online my booking said unable to change. I called back and that operator said it was it as locked from the attempted changes and said try again and it had released. I asked them to stay on the line while I put the change through so I wouldn't have to call back if it failed. It went through and the POUG popped up instantly at which point I explained about my original call and he confirmed the two charges.