Ringy1, I think your boiler is a Minute EHE, I could not find a Minute A. There also is a 25HE model, so indicating which one you have will make assist in giving correct advice.
Your boiler is a system boiler, so it will heat radiators and also water in a cylinder. Therefore, if your guy fitted two motorised valves, (one for HW and one for CH), there is no need for a relay board or a relay. On the other hand, if a Y plan has been fitted, then a means of telling the boiler to get working with 240 volts will be needed. A relay is not the only way to do this by the way!!
If motorised valves have not been fitted, it would be prudent and economical to do so.