As I understand it, the 66th MTB Flotilla was created for an anti-submarine role and its boats shipped 32 depth charges in place of the torpedo tubes. This photo shows my father's boat (731) with the depth charges in place and what are (presumably) some of the the other boats of the flotilla alongside. I don't know the location though - might it be Portland?
I have transcribed a Report of Proceedings from the National Archives relating to an anti-submarine patrol carried out in the Channel by MTBs 731, 747 and 750 on 23/24 December 1944 in which an attack was carried out on a suspected contact - although as no U-boat is recorded as sunk in that location, it must not have been successful. The report concludes that "Intelligence showed that there were some U boats in the area, echoes were loud, sharp and distinct. Extent of target was approximately 8 degrees at varying ranges between 750 yards and 1200 yards. No movement or doppler were however recorded and QH2 showed the target to be stationary. Recorder races were good."
Hope this is of interest.