So many years since I've had to deal with this kind of thing so I have no idea how things work.

I have commercial vehicle insurance with protected bonus of max years (and then some!)

I am covered for damage due to vandalism, without loss of ncb. I read that to mean irrespective
of protected ncb.

Plagued by local smackheads damage by attempting to break into or even steal the vehicle.
Latest:- chewing a rear door lock barrel to the point of 80% missing, damage to rear door due jemmying
past secondary security deadlock, damaged cab fuse panel cover and possibly the panel and wiring itself.

How much, if at all, is such a claim likely to affect future premiums?

Might seem a daft question but I'm sceptical about these insurance cos and "without loss of ncb" seems
rather irrelevant to what else they can do?

-----

Just to complicate matters, renewal is due this week and, as before, existing insurer are seriously trying it
on (+30% !)
Experience tells me that's just a try on and they'll reduce it - but I'm being quoted less than last year by
several others, incl. major companies so likely to bail out right now.

Which raises more questions about what I have to declare to potential new insurers.

Sadly it's different per insurance company.

If you wish to play about with figures on comparison websites - not for the purposes of getting a false quote obviously - make sure to use your correct address but a false name.

This will give a truer reflection of what the hit will likely be, without linking it to you.

I never stay with the same company after the first year unless their first offer for the following year is a reduction in premium.

I don't play games with insurance companies and if they want to give an inflated premium it's their loss.