Incredibly sad Bob, although I must admit I no longer give these people the credibility of referring to them as travellers. My preferred terminology cannot be shared here. I doubt you will get much help from the Police either.
At Kirklees we use a retired shipping container with three large locks on the door protected by steel covers to prevent bolt cutters or grinders being used on them. Even then one day a thug steals a motorcycle, leans it against the side of the container and sets fire to it. The transfer of heat through the steel side still did a lot of damage.
Nowadays we have a completely ineffective Police force and a totally inadequate criminal justice system. The thugs have never had it so good.
Good luck with the insurance.