Dunno what the hole size is on the current boss - IH has not failed yet...(since the last time) - but that might be a way to go.
BTW - only standard boss I've found is 25 quid from BES - does that sound about right? - or anywhere else to get one..
1969 built house with stainless steel plumbing and direct DHW cylinder (48" x 14") with unused backboiler. Water heating is by immersion htr only - but the type seems to be unavailable now (Dunlow - Duncan Low Ltd - list no 107). It is flanged and mounted by two bolts - size of flange is about...