Intrestingly, the latest "British Warships& Auxiliaries 2010/2011" says they are expected to be decommissioned in 2010 and replaced by the ATD 2909 class SD RELIABLE & SD BOUNTIFUL that are both now in Portsmouth, perhaps thats why they have lost their prefix.
Another mystery, if Serco have bought out Denholm, why do the new tugs have the SD prefix?
Regards
Bob