I gave upon paraffin in the late 1970's after having a nmber of "sooty" episodes, and failures, compounded (or caused by) being away from home on occasion for too long on business.
Then I used Autoheat tubular fan heaters through the 80s and 90s, with various problems, mainly associated with thermostats giving up. I had a major disaster in the winter of 2001/2002 when again a thermostat failed, and I lost about half my collection to cold and disease - again because of working away from home.
I have gone a bit overboard as a result, and in a rather larger greenhouse have two Biogreen 3kw tubular heaters, each with high quality digital thermostats, which are set at 7C during the winter - I found last winter set at 5C too cold for some of the plants I grow. Each heater is wired into a different ring main, and I have a 4kw propane heater set at 5C as backup.
With a greenhouse your size, you won't need that much heating capacity, and only you can decide how much redundancy you want, but I would recommend for cost reasons a good quality thermostat. Mine came with the heaters, although are separate units. Ideally you should have one where the hysteresis (the lag between on and off) is smnall, like 0.5C. I have heard good things about Parwin, although they seem to have stopped production of new heaters, but stcoks are still available I believe. You wouldn't need more than a 2kw unit. I line my greenhouse with bubble glazing, which I keep up all year round. This does reduce heating costs, even with Economy 7 rates.
At the end of the day its a personal decision really - the risk to your collection vs the costs of installing and running th eheating system.