Well, I managed to get my Apps in the end. I am still deciding about the final one to choose, as, to be honest, non of those on offer are very good. I should have chosen 7 from about 8 games and 8 utilities, except that some of the "utilities" are no more than games anyway, and not all of them are compatible with the Vivacity!
The way around the going in circles problem turned out to be: don't try to use the link given in the text from T-Mobile, but instead type it in manually into a browser. Simple, when you get round to thinking about it. Why the supplied "live" link in the SMS goes to a fault page, while typing the characters into a browser works, I have no idea.