The Bulleid coaches would have been probably gone by the end of 1968. From the mixture of stock this is likely to be a Channel Islands Boat Train to Weymouth.The building with the tower is the Civic Centre (Town Hall etc). When developers built a new shopping centre to the right (as seen in the picture) they managed to put a couple of support piles through the tunnel roof, closing the main line for some time. It seems the developers didn't realise that the two bits of railway line on either side of their plot were actually connected.

The SR codes list I have seen dates from the 1970s shows Channel Island Boat Trains as 90 with 96 for Southampton Western Docks Boat Trains which this and the similar D65xx & Tadpole picture could possibly have been.