Hans,
Technically and legally, the relevant tax authorities are obliged to pay interest if they have not refunded your VAT within 6 months. In practice however, this can be very difficult to obtain but if the amount is significant and the delay is considerable it is always worth pursuing. I have found the UK tax authorities to be prompt and efficient (it was some time ago - they may have a backlog). Spain and Italy can take a lifetime! (and they dont pay interest).