Re your coking up, some of the “refurbs” on eBay have their sintered evaporator damaged by over enthusiastic decoking of the sintered evaporator, this can lead to rich running and early coking up, especially if the burn rate has not been checked and set using an exhaust gas analyser after install when the exhaust and air intake resistances are a known. Personally, I would never touch an evaporator except to remove loose carbon and if heavily carboned always replace them. As the cost of a new burner cartridge almost equals the price of some of these “refurbs” it does make one wonder. I have been asked to install these for people who have bought them and politely declined on every occasion as I don’t want unnecessary agro down the line. However, the age of your diesel will also be a factor, possibly a big one. I have had brand new units coke after a week and ditto the replacement, installing a temporary tank with decent new red diesel to EN590 proved the point in each case. I honestly think it is a really good idea to install a 25l tank which is kept topped up so fresh diesel is always available, more so where the tank is a long way from the heater as the fuel pumps for all these heaters need to be close to the tank as they are “pushers” rather than “pullers”