I've had a stomach bug which has set my AF and pain management back (to say the least!) but I'm experiencing a dead leg, numb-ache type sensation all down my legs which emanates from my bum. I've not had it before. Any one else?

Yep... When i first had it the pain was going down my legs, around the front to my abdomen. Do you have a fever? Are your lymph nodes in your legs swollen... All this could make the pain worse.

The End is Nitrate, the only times that I get my legs like the way you debscribe is when I am low in potassium, diarrhea can cause your potassium and electrolytes to be depleted.
Just in case have you potassium levels check and drink pedialyte, this no only beneficial for childrens and infants but for adults.
Hopefully is nothing but just another nasty side effect of fissure spams.