I have just bought a rx7 fd3s twin turbo automatic and on tickover the stock oil pressure gauge shows quite a low reading. When you increase the revs the gauge increases quite quickly to the middle reading, is this normal?

 

It's a common issue. The sender for the oil pressure gauge is located just beneath the oil filter pedestal and has a single wire spade connector. That connection can get fouled being so close to the filter during changes. The signal is weakest during idle and can't get through that dirty connection. Especially if the oil is thinned already. Remove it, clean it well and replace it and see what you get.
Sometimes the senders do go bad and need to be replaced.