Oil Filled Heaters, any one recognise them

I don’t think they are oil filled.

they look like vertical water filled radiators with a heating element option that makes them electric radiators.

if that is the case, they should be filled with water and a corrosion inhibitor as per the instructions.

As per James post.
It's a 'dual fuel' radiator that can be plumbed into central heating, or used with an electric element, or indeed both.
Such as this (not your exact one):
The element can be replaced if there's a problem with it
(just an example) SIM - Electric Element 1200w Standard - https://ukradiators.com/sim-electric-element-1200w-standard.html

The rad should be mostly filled with water, but not completely. With a 1200W element you might expect to hear kettling noises - I'm not sure if this is what you mean by pop and fizz?

If in its present state you are happy to run it for a while, see how high up the 'fins' get hot. If they're hot to the top, or near the top, you have enough water in it. If only hot half way up, or less, it needs topping up!
Is it leaking? If from the vicinity of the element, that definitely needs replacing! I had an instance where the element developed a leak that put water inside the cable which eventually leaked into the fused spur and blew the fuse!

An electrical test of insulation and current drawn would help establish if there's anything untoward.

If you are competent and feeling strong, you could disconnect the cable, turn the radiator upside-down, undo and withdraw the heating element and inspect it, replace it if necessary, and also check the water level.
If going this route, I would recommend fitting an element that has an integrated thermostat, which can maintain a more even temperature (unless you need to run the thing flat out anyway?)

Many thanks for taking the time and trouble to reply
sorry for the delay, I have been fighting with a 9 foot robot, he is old and incontinent and blows fuses !!
I do see a fair few water filled rads with ancillary elements, usually when the property has an Aga or oil burning boiler, useful for the summers
but i have never seen a water filled one, as to my old fashioned way of thinking a standalone unit would be a pressurised vessel, otherwise known as a bomb !
I managed to lift two of them of today, and at the top they have a small vent hole, with a I presume a safety valve, and traces of mineral oil are evident around that area
the client has contacted the architect to see if they specified them, frankly they will have been the cheapest they could get going by the rest of the electrical equipment and the shoddy install
they are controlled by a timer/thermostat unit, so if i cannot get any info I will obtain some replacements
in another building I notice that all of the heaters have been replaced for quality German units, the development is only 3 years old !!
will get back to you once I know a little more
meanwhile off to fight with Franko the robot

Jurassic Sparks said:

i have never seen a water filled one, as to my old fashioned way of thinking a standalone unit would be a pressurised vessel, otherwise known as a bomb !

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Thats why they are not filled full - there's always a substantial air pocket present. Hence my comment about not filling it full! I should say I've never seen an oil filled one of this style.

The small vent hole is a bleed valve for when the unit is used as a traditional radiator.

If you discover that the unit actually has oil in it, please let us know.

Jurassic Sparks said:

will get back to you once I know a little more

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We would love to hear the outcome