This thread has re-appeared due to the post above this (which has been removed). I'll add that Analogue Seduction sells SME parts, including oil, damping fluid and the HTA key, or an e-mail to Elaine Wratten at SME (sales@sme.ltd.uk) will also get a quick response.
However, that key is a ludicrous price (currently £47.98) so I would suggest a torx driver from the local hardware shop would be better, if you can find a fit, as it is a non-standard size. The SME key is 3mm from point to opposite point, which is between a T10 (2.74mm) and a T15 (3.27mm).