what usually happens when they cannot check for supplementary bonding visually?
It requires slightly more effort/work from a testing/verification perspective (hence more money spent) and will no doubt require removable of bath panel, light fitting, etc, all of which could result in cosmetic damage, to physical check for presence of bonding and to check it's integrity.
I presume people here have generally agreed that zero RCDs is mostly a C3? Does the number of RCDs affect the rating?
Based on what your electrician has said, then RCD will be required for lighting circuit in bathroom, if bonding is not sufficient. Catch-22 - will either need to confirm bonding is in place and sufficient if not can be made sufficient (costs involved) or have RCD/RCBO for all appropriate circuits (costs involved).
If he grades according to guidelines, surely it's not his fault if he's followed that? Have people here had to deal with legal issues when something bad happened even though they followed guidelines?
Thankfully not been involved any legal issues to date. Would hate anything bad to happen, hence previous recommendation to give you that extra piece of mind (assuming you can afford it).
On a separate note, if not already done so, would strongly suggest you also consider having smoke/heat alarms properly assessed (fire bridge can do this for you) to see if they are adequate and meet current regs. - One area where as a landlord you want to fully up to date, even it is going to cost extra to bring/keep you fully up to date.
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