Simon, best to go ask at your local immigration office. The rules (or at least their implementation) varies from office to office. When I got my 60d TR VISA extended last time (extended to early April) the officer told me I would probably get another 60d extension in April as the current rules are due to run until end of May, as
@Ben ******
says above. You can apply for VISA extension within 30d of expiry so I suggest you get your ass over to HH Immigration sooner rather than later to find out for sure. Can you report back here what they tell you?