Having LGV C1 under grandfather rights (pre 97) and driving that commercially means you must have by law a current valid driver cpc card (DQC) where you attended 35 hours of periodic dcpc training
To obtain LGV C you will need to pass the theory module 1a and HPT module 1b as well as the practical on road/reverse test module 3
No need for driver cpc modules 2 and 4 tests
I would also do the C training with a school that has the practical training approved for driver cpc hours so you can get 14 or more hours towards the 35 hours you need for your next DQC 
The forum sponsor (Peter Smythe) has a deal on this I think