Got to get up to get down

What is the correct etiquette for giving up your seat? We had a right old game of musical chairs on the Tube the other afternoon. The walk from Northern line to Central line at Bank is a ridiculously long one followed by loads of stairs up, and up and up by which time my legs were feeling decidedly shaky. I've been very healthy this week - I've been in Moscow so it's probably the vodka - but when my leg, especially my right leg goes numb it's a pain in the arse. Well...slightly lower to be more accurate. So it was a relief that I could find a seat on the Tube. Then some old lady with a walking stick gets on, so I feel obliged to get up. Next stop an even older man with stick gets on. Two people simultaneously get up for him leaving me with the chance to take one of the newly vacant seats. I get a slightly dirty look from the now standing youth that was being generous to the old man with stick but not to the middle aged man with shaky legs. What are the chances of this though...? At the next stop an even older lady but without a walking stick gets on. She stands staring at me...now...and here's the dilemma...is she old enough to get the seat? Do I have enough MS to keep the seat? Is my MS visible enough to let other passengers know I'm not being a selfish bastard. As the door starts to shut I can take it no longer...I get up...the old lady gets the seat.