When are my melons ripe?

I was given a little melon plant a couple of months ago which has grown really well in a cold frame up the allotment. It has three fruits on it now and i have pinched out all other flowers. The rate in which they grow is suprising but the first one that set seems to have either stopped or slowed down considerably. It is about the size of a small grapefruit and still quite firm. How do i know when to pick it?

Totty

Someone asked this recently and the answer given was when the opposite end from the stem is very slightly soft.

I think this is the standard melon test , if the end of the melon moves a bit under pressure from your thumb it is ripe, rock hard and it`s not 

Thanks all!