The-Snowman:
Provided no passengers then yes you can.
Generally no, you can’t drive a PCV using a Category C licence.
The old concession that Dazza1980 posted about, detailed on the now withdrawn INF52 leaflet, was withdrawn at the time the Third Driving Licence Directive was implemented in the UK on 19 January 2013, as the previous concession was not compliant with the new Directive. This is why you won’t find anything about the old concession on the driving licence part of gov.uk today.
However, the changes do not affect any drivers who enjoyed the concession at the time of the change, so any driver holding category C on a GB licence from or before 18 January 2011 can still drive PCVs under the rules outlined in INF52.
The old concession is so limited that there are probably very few drivers who knew about it. The number of drivers regularly using the concession was estimated to be very low at the time of its withdrawal.
The-Snowman:
Cat D is for buses that are automatics. For drive manual coaches you need a cat C but to take passengers you also need a pcv license.
Category D allows you to drive all buses and coaches. You never need category C to drive a bus or coach.
Automatic only entitlement is shown against a category in the normal way - with a 78 restriction code.
If you have D(78) entitlement, then you are limited to driving automatic PCVs.
Under the latest rules, the 78 automatic only restriction can be removed by exchanging your licence if you have or acquire manual entitlement for any one of B, BE, C, CE, C1, C1E, D, DE, D1 or D1E because you passed a test in a manual transmission vehicle and are not limited to automatic transmission vehicles for medical reasons. The majority of drivers hold manual B entitlement, so can remove the 78 restriction from all vocational categories held by exchanging their licence.