Finishing a session at gym some 3 weeks ago and felt something go back of calf about half way down.

No problem initially but did it gain pace days later,could not bend my knee. Pain now back of the knee ,difficult to bend it still

Was advised muscle,maybe,but damned painful .

any ideas anyone Cheers

Is it swollen or enflamed (hot to touch)? Is there any deep bruising visible?

Calf muscles extend ankle so I doubt it is calf muscle since you cannot bend your knee.

Is it swollen or enflamed (hot to touch)? Is there any deep bruising visible?

Nothing as I can see,but a couple of weeks ago was impossible to walk on with pain,bit better now but still pain back of knee

Well, by now, swelling or inflammation, which usually accompany a tear would have reduced or disappeared. But if you didn't notice any obvious swelling at the time, it doesn't sound like a muscle tear or a badly sprained ligament or tendon.

Bursitis maybe...?