At last the matter has been resolved.
Had a call on Tuesday 9th April from the manager of the complaints dept.She was very nice and apologised profusely saying that this situation should never have occurred and further steps would be taken with the people involved.
Sounds like somebody is for the high jump!
Anyway,we were told that backdated funds should be in our account on Monday and paperwork should be received in the post the same day.
I checked our account today and the money is in there = yippee ! Postman also brought the brown envelope.
It's such a relief to finally get this sorted.