New tenant being difficult with plumber

Maintenance

I've got a new tenant being a bit difficult with my plumber...

Their [new-ish] boiler apparently occasionally goes into fault mode with the lights on the front flashing and needs resetting. The plumber has been in direct contact with them and has asked for a video of the flashing lights (as they are a diagnostic code), but the tenant has said 'they're just flashing - its all the same'. I've been in touch with the tenant and asked if we can get access to look at the boiler and he's replied 'we're both at work'...

How much trouble do I need to go to here? If they're not willing to help us sort the problem (which is only intermittent), am I obliged to have multiple plumber call-out s (which costs me) to diagnose and sort the problem, both of which might need to be at times to suit them (they both work shifts so sometimes awkward times) - should I be specific and say he's coming at xxx time and if you're not there we'll let ourselves in.... or should I just put it back on them and say they need to help out or its not getting sorted?