How do I find out if a calibration certificate for a speed camera has expired? I cannot afford the points on my licence and therefore want to know if there is a way I can avoid penalty points.

You are entitled to see the calibration certificate for a speed camera.  This should identify the make, model and serial number for the camera and prove that it has been calibrated within 12 months of the alleged offence.  It may be that the Police refuse to disclose this information before a Summons has been issued and strictly speaking, there is no obligation for them to assist before then.  However, most Police forces will provide information upon request.