And please do read the information sheet that whammel has mentioned.
I have been using the Blue Pals 50x50 electrodes ever since Odstock first set me up with my FES. Never had any reaction from them. And the Odstock service is great - when I am down to the last pack, a quick phone call will have some more through the letterbox within a very few days.
I will also put in a plug for Joe Green (author of that info sheet). I had my last appointment with him, and he spent 10 minutes tracing an intermittent fault - right down to a break within the moulded anti-strain bit on one plug. Very thorough, very efficient. I hope to see him again next month - but then they are all nice people at Odstock.