I have a massive sum with them, planing to visit them personally; if anyone can give me a hint where to go, let me know.
If that's really true, you need a lawyer in the UK filing statutory demands and threatening to put them into bankruptcy.
Their registered address is:
BITSTAMP LIMITED
5 NEW STREET SQUARE
LONDON
UNITED KINGDOM
EC4A 3TW
Company No. 08157033
I just sent a note to the building owners asking if they actually have office space there.
(They don't owe me any money, so I don't have to sue them. You may.)