Help....I need your advice please about greenhouse heater for Mum!

There are two reasons to heat a green house, one is to protect plants from frost, the other is to grow tropical or tender type plants and to provide a higher level of heat. Growing tropical plants is expensive to heat particularly with electric.
If she wants a heater to cut in when it starts to get cold there are a lot on the market, just google Green house heaters, i would advise one with a fan to distribute the heat more evenly and to run on fan only in high summer temperatues to cool things down.

Try this..... Tropic 2000 Electric Greenhouse Heater


If your mother is eldelery I presume she would not want to be messing about with parrafin, trimming wicks and filling up etc, also they are on all the time with no thermostat. It might be an electric one set to just over freezing temp that willl only come on when things get quite cold is the best bet.

Big savings can be made by insulating your greenhouse with bubble wrap or making a smaller area within the green house and just heating that.