thanks for all the suggestions!
As Greg says the Seychelles are no longer an option. Not sure about Mauritius - I thought the weather was not good in July?
I did really fancy going to Grenada but BA flights are on a Tuesday which is out for us( Tuesday and Wednesdays flights not possible)However, we may yet decide to go there and use the voucher to get to Barbados. We have been 3 times to Barbados and could easily go again (but I would like to try a new destination.)

yes I would like to go to Hawaii( good memory fbgdavidson!) but not possible on the Amex voucher.
St Lucia may be a possible.
Cape Town is def. in our sights but we are looking to go in April some year!
Dubai- anyone been in July/early August? I would worry that it was too hot!
We love Bangkok so that is another possible.
Another possible is Antigua.
Also we could look at Washington, Baltimore , Detroit and Seattle and investigate internal flights from there.
Matthew- we would like some nice weather- I love both City breaks and lazy, sunbathing type ones. Although with the sunbathing I HAVE to have trips, site seeing etc. Not interested in skiing!
Finally- Manchester! Hmmm, will have to think about that!!