So Ollie is only the second one to have leg trouble? Why not have a blood test done at the vet's to confirm it's Marek's and not a bacterial infection in the spine that's affecting his legs. Just to rule it out as it is indeed likely to be Marek's, but the symptom of leg trouble is now less common than it used to be. Ours hardly ever showed the leg problem when they became ill. You never know, it might just have been bad luck.
A blood test might give you some other results as well, I found it useful.
Do you have a good chicken vet in your area?
We have Mareks in one of our flocks, unknowingly brought in when we bought some youngsters last year. I bought 6, of which 2 developed leg problems within weeks. One was culled as it didn't improve and he was a surplus boy, and the other did get through it as it was initially thought to be a bacterial infection in the spine and she received antibiotics which got her through it This was before the results of the blood test were back and we found out about them carrying Marek's. Unfortunately she was taken by a weasel only weeks later
so we'll never know if she would have been affected by the Marek's or not (the vet's notes mention that she wasn't 100% convinced that the leg problem was definitely caused by the Marek's).
The rest of those youngsters were all boys and hence went into the pot without ever having been ill so we'll never know about them either.
We keep our Marek's flock, which consist of about 15 birds of the same age as yours, and the others well apart. We change boots and clothing, use Virkon S for disinfecting coops that are being moved etc. What I found most important is to keep stress levels down - we had a bird become ill every time there was a major shift in the pecking order (young POL fell ill after she started being chased by a hormonal young cockerel, who in turn got ill when he was deposed by his younger brother who grew much bigger than him, things like that). I now regard them as a separate flock, no other bird goes in there but I'm not culling them either as long as they are doing fine and they are very lovely birds. We try to keep their immune system as good as possible with quality feed, greens, poultry spice & tonics etc, give medication when they fall ill and have them put to sleep or cull them if they're not getting better. Over the years they'll pass away and become a smaller and smaller flock, until they're all gone and we can disinfect the whole area and start from scratch.
Hope this helps