If they are only 10 years old it is probably not worth replacing them, unless one or more is underpowered.
electric heating is quite expensive but storage heaters are cheaper to run as they use cut-price overnight elecricity
the trouble is that they heat up during the night and lose heat to the room during the day.
If you are at home all day, this is quite convenient, especially if you get up and go to bed quite early, as many retired people and parents of small children do.
they are not much good if you are out at work all day, because by the time you get in they have given out most of their heat.
They usually have a shutter to prevent or slow the release of heat, and if you are out, you can shut this until you get home in the evening. It is often operated by a turn knob.
They also have an "input" control or power switch to increase or reduce the amount of heat that is stored. You usually turn it towards max in cold weather and to min in mild weather. And turn them off in summer when heating is not required.
You could get an instruction booklet and study the controls. Many of them are (were?) made by Dimplex which is part of the Glen electrical group, based in Ireland.
If the tenants find the heat insufficient, I'd suggest a small oil-filled heater for the bedroom(s) and an electric panel heater, or large oil-filled radiator for the living room. A long room needs two for even heating.
in a modern well insulated home I find an 800W oil radiator adequate for a bedroom. A living room needs more because it may be twice the size and you won't spend all your time under a duvet. The larger ones have 1/2/3 power switches, and a thermostat. Around 2500Watt may be suitable.
My preference for oil radiators is because they give a continuous moderate heat, and get no hotter than a teapot, so will not start a fire even if you drop a newspaper on one, or a curtain blows against it (other portable electric heaters are not so safe).
They are cheaper in Spring, when retailers are clearing their winter stock.
If the bathroom is cold you can get a wall-mounted heater or a heated towel rail (these give out very little heat, especially when they are wrapped in a thick insulating layer of towels). There are fan heaters made for bathrooms, they have to be fitted out of reach and away from the bath (the electrician will know the rules).
All electric heaters are equally efficient, in that they take in 1kW of electrical energy and give out 1kW of heat energy, and it costs 17-22p or whateber your current price is. You cannot get out more heat, or use less electricity, in a more expensive heater, although the adverts may try to make you think this is possible.
You can find some example prices on this wholesale supplier. They will sell to the public with VAT added. Supermarkets, DIY sheds and places like Argos and Currys all sell very similar heaters. I bet most of them come from the same factory in China.
I wouldn't get the ones with a built-in timer; this is about the only moving part and the only bit that goes wrong. They tend to get noisy with age.
Also, check that the windows fit well and have thick lined curtains and/or blinds to cut heat loss. I am not a landlord but I imagine after 10 years replacements may be a tax-deductable expense.
Beware of damp. The rooms, especially bathroom and bedrooms, need to be ventilated or they will accumulate damp, condensation and mould.