Thanks for the responses.
Unfortunately no spare PCI-e slot (only have one PCI-e slot x1).
Not had the problem before inserting the card (or after since removing).
As for the PSU - it's a Corsair 450W VX Series PSU - ATX12V v2.2. I would have thought that would have been ok wouldn't it? I've got a couple of fans in the HTPC so wouldn't have thought heating would be a problem.
Mmmmm.....not so sure....