I have just been through all this with the new truck.
I did all the trawling around, with insurers who advertise specialist 4x4 or modified cover that I have used before like Graham Sykes, Flux, Knotts etc but they were all coming in at £1000 plus which I was surprised as I am a normal insurer with no startling issues like poor driving record and I have about as long a NCB as its possible to have. I questioned this with each of them as I knew I could get it much cheaper elsewhere (and not trying it on as I really could.) And they each gave some excuses about rising costs etc.
Ended up on Go Compare who came in with an insurer at about £300 which I then loaded up with the usual protections so totalled about £350.
Must just add that I will never use Flux again. On my last policy with them on the 100, I was impressed with them so called for a landlord insurance quote on my rental property. I then received a call back from the motor section saying my existing truck insurance premium had a surcharge of an extra £100 as being a landlord was a second occupation and raises my motor policy costs, if I didn't pay then the policy would be cancelled.