Hi Symphony, Thanks for the reply. I should have explained that when the boat was refloated after the engine fitting, we had to call the engineers back as there was a leak at the clamping ring. this was dealt with, not with sealant as apparently this is frowned upon by Volvo but I guess by retightening the bolts. There is however a remaining "weep" ( as I understand is often the case). I was wondering if this could, and I think it could, cause a current to flow, hence me wondering about linking it to the anode. If this couldn't occur, then you are quite correct - it's just rust!!.
Bill