What is the difference between a slow burn fuse and an ordinary one?
I fitted a brand new compressor last July and I have just found that the 30amp sbf has blown. Have fixed up an ordinary fuse as a stop gap and the compressor is working ok. Any suggestions as to cause of fuse blowing?

A slow blow fuse is designed to withstand the inrush currents you get when motor starts to turn.
All fuses can become 'tired' and just blow from old age sometimes, especially if the current flowing through them is close to their rating.