Thank you for using Doctorspring. I understand you are unhappy that your father's problems are not being addressed properly by your GP. Considering your father's age and pain in knees predominantly, it is the type of arthritis called osteoarthritis due to loss of bone mass. If your father has been having arthritic pain since earlier in life or is also there is also pain in other areas of his body, I recommend you to visit a rheumatologist.

The main stay of treating osteoarthritis is physiotherapy and lifestyle changes in addition to analgesics. Your GP may not have prescribed your father Diclofenac because of adverse drug interactions with other drugs he is taking or due to other health issues he might be having. Do not administer over the counter drugs especially to older adults and kids. Instead you could speak to your doctor regarding other drug options taking care to explain to him that your father's pain relief is incomplete.If your father is physically active and his bone age is young, you consult with an orthopaedician regarding options for both knee both replacement.

I hope the information is of use to you. Do emphasize your father's inadequate pain relief with your GP and ask him about the availability of physiotherapy.