Dave
Sorry but I also don't know anything about knee arthritis. However, there is almost certainly someone on here who does
I have a somewhat dodgey left knee that can give me a bit of pain if I put too much strain on it at the wrong angle. I'm training for LEJOG and like you I was a bit apprehensive about getting back into cycling. However, I have now done over 1,000 training and the knee has been fine. In fact I would say its stronger and less prone to pain that before when I started.
From my experience I would suggest you build up very slowly. Start with a short trip of 2-3 miles, see how the knee feels and if everything goes okay slowly increase the distant, allowing plenty of rest days. Stay away from steep hills at the start that could put strain on the knee and don't be worried about getting off and pushing the bike up any hills you do have to go up.
If at all in doubt consult your GP.
Best of luck.