Took the K-m to camera shop today where a very experienced owner gave it a good going over for 45 mins, outcome was the problem lies with the camera body receiving info from the lenses. By trying to wiggle the lens he managed to get readings from F4.0 to F8.0. Repair to mounting ring (if available) will be about £130 - £150 so will settle for a new body, just have to decide which model. Thanks for all your suggestions.

Hi John, thank you for your input I will run through your suggestions tomorrow and hopefully resolve the problem.

Best regards, Andy

HI Kris, yep first thing I checked and cannot seem to change it in Auto or Manual, like I said I am not overly tech savvy but thank you for your reply.

I have a Pentax K-m and whether I use my Pentax18-55mm, 50-200mm or my Sigma 10-20mm, 70-300mm and 150-500mm lenses the display screen recently only displays F4.0 regardless of the light conditions. Therefore some photos I took outside on a recent walk on a very sunny day were greatly over exposed (was using the 2 Pentax lenses at the time). How do I go about changing the F stop. I am not greatly technically ok with all the workings as this was a retirement gift which I have been using for a few years without any problems. Anyone any ideas or do I just go for a new camera as the K-m is now 12 years old and well used.