I have an oil fired cooker boiler with an insulated flue. The flue is leaking water apparently from between the skins. As the flue has been stripped down and refitted several times due to cooker move and replacement I suspect that it is "past its sell by date" and needs renewing. Any recommendations as to the best make (existing is Negarra) and do I need to have it installed by an Hetas chimney fitter?