Rhubarb - when to stop cropping

I gather the rhubarb season is coming to an end (I'm in Scotland BTW). I have four clumps of rhubarb which still have healthy and delicious stalks on them. Is it too late to pick some of them? Should I leave them now or can I have that final rhubarb crumble this weekend?

 

You'll notice when the plants start getting tired, just fed one of mine to see if it'll pick up again but its starting to look like it wants a rest.

the one i've been feeding regularly on the other plot is still going strong though.

If its still producing well then i'd go for the crumble. Tesco have been selling custard for 6p a pack, so i've been nearly living on the stuff.