I've found more info that might help.
The Assembly Rooms Theatre was built in 1868. It opened as a music hall. In 1894 it became an ‘all purpose’ hall. The proscenium was 17ft wide and the stage 17ft deep. Silent films were screened in 1909 & 1910.
In 1920 it was sub-divided to provide a dance hall upstairs with the lower part of the building in various entertainment uses. In 1940 the building suffered some damage from German bombs. It was later restored and in the 1950’s until 1960 it hosted some pop concerts.
A supermarket was created in the lower level and the entertainments moved upstairs. In 1987 it was in use as a community support centre. It was demolished around 1990.