Once a week I do a 10 mile cycle around a very technical trails/mountain bike type track. It's hard work and good fun. Although I do enjoy it I am wondering if I am actually benefiting from this in terms of weight loss and fitness. I'm not fat, just trying to get trim for summer. Is it enough?

i would imagine so, coupled with good eating habits and other lifestyle choices!

Better than nothing but only once a week isn't going to help too much. You need to do it at least 3 times a week to notice results anytime soon. This is also highly dependent upon your diet and metabolism.
When I cycle, I do about 15 miles in a very hilly area...4-5 times a week. You get into the groove and it does get you into very good shape.

It's a great way of keeping fit, just add to it developing other parts of your body and as long as it's fun I think you will do excellent.

maybe twice