Hi Folks, I need your help (again...)

I am trying to arrange Trailer Insurance. It is for a Brian James RT4 covered trailer.
But - it gets complicated. I would ideally like to have cover for the little track car that "lives" inside the trailer.
So it is really insuring the trailer and "contents". Is this something that is ever offered by insurers?
Note: I tried A-Plan as my track car itself, which is road legal, is insured through them. However, they wanted to cancel the existing policy, start a new policy and add the trailer to it. This was eye wateringly expensive, they would not give me much back for the 1-month old existing policy, and they cannot cover the cost of a the car when it is being stored within the trailer itself.

Many thanks! Current: L322 2011 Zermatt Silver TDV8 Vogue SE, MB GLB AMG35, Renault Clio R200 Cup
Previous: Audi RS4 Avant Nogaro, Audit TT Sline, BMW M3 E92, BMW Z4 3.0i SMG, Macan GTS, Boxster S 986, Touareg V6 TDI

I'd have expected the car to be covered by its own (existing) policy. Perhaps you can change the answer to 'where is the car parked overnight ?'

I went through this with a boat - my car insurance only covers the trailer I'm towing 3rd party, but I clarified with the boat insurers that they covered damage to the boat whilst on the trailer, even if I cause said damage.

Why won't A-Plan cover it ? Do they view it as easier to steal ?

Thanks for the feedback Alistair,
A-Plan would not alter the existing policy and add the trailer.
Since the Howden takeover I wonder if they have performance targets based upon new business placed…
They offered to cancel the policy, giving me very little for the remaining term, and look to create a new policy. But they told me no insurer would underwrite a vehicle in transit or vehicle stored within the trailer. The trailer has data tagging hitch lock, multiple bulldog wheel locks etc.
Google gave me a list of specific Motorsport insurers who seem to offer cover for trailers and vehicles in transit.
I will report back on how I get on. Current: L322 2011 Zermatt Silver TDV8 Vogue SE, MB GLB AMG35, Renault Clio R200 Cup
Previous: Audi RS4 Avant Nogaro, Audit TT Sline, BMW M3 E92, BMW Z4 3.0i SMG, Macan GTS, Boxster S 986, Touareg V6 TDI