It is a requirement of the Money Laundering Regulations 2007 (2007/2157) that individuals "who by way of business" carry out trustee duties for any trust (naturally, including pension scheme trusts) must be registered with HMRC by 1 April 2008.
Sanctions for non-compliance include fines and even imprisonment.
The position of professional trustees is clear cut: they must register. However, a grey area exists in respect of those trustees who receive payments for their duties in relation to one scheme. Depending on the nature of the payments, it is possible that they might be considered to be acting as trustees "by way of business". Professional advice should be sought in cases of doubt.