Thanks for that Red onion. Can I ask why it's not efficient for oil to heat stove? Just curious cause it makes some difference!! Yes I'll have to be shown valves also as all zones come on automatically also which defeats purpose too. Is it safe to use oil if plumber doesn't get back til new year do you know?It's not efficient to have the oil heating your stove, but it should be safe.
There might be a valve added to prevent it happening. No harm getting the plumber to look at it, and while he's there get him to show you where the pressure relief valves are for your own piece of mind. These are a safety feature to automatically open if too much pressure were to build up in the system.
What is a heat sink as opposed to a radiator. How can I tell if heat from stove goes to oil boiler? Plumber said he'll fix it but is it safe to use solid fuel as well as oil until New year?Going from what you have told us it is not dangerous, however it is not desirable for the reason, that (I am assuming) the stove is vented and therefore may act more as a 'heat sink' rather than a radiator. Also you have no control over it.
Plus if it's happening the way you state it is, it's also reasonable to assume that it is happening the other way as well, ie. your stove, when burning will also heat up your oil boiler, this you definitely do not want (wasted heat).
Usually the fitting of a non-return valve(s) or such like will be a simple fix, but IMHO should have been done from the start.