Thank you Vim Razz,
I have been trying something along these lines but still not having much luck. With limited power and fuel my rescue vehicle's orbit is much smaller than the target's, so whilst it can orbit many times during a single orbit of the target vehicle, it never seems to give me a good rv. Perhaps it's just trial and error? I shall have another go with your suggested method tomorrow. Are there any 'tricks' like when going to Mun I was told it was best to wait till it was at about 90 from the intended intersect point? Does that work with this kind of thing?
If I miss the time when the target is at its periapse, I have to wait a whole other year or at least, many months.
If I try and make my orbit match their's (someone recommended ont he KSP facebook page), it of course takes about 2 minutes burn and by the end, I don't have much at all.
Thank you for your help and patience.