Your responsibility on this is absolutely zero … unless the light globes were of a type that a reasonable person couldn't change without professional help.
If it was so difficult, the tenant could have contacted the agent or yourself. That they chose the easiest route – and the most expensive one – was their call and not yours.
This is what Fair Trading says to Tenants about light globes:
You are responsible for doing things like replacing light bulbs, changing the smoke detector batteries, cleaning windows, dusting, removing cobwebs and routine garden maintenance such as watering, mowing and weeding.
That's my bold italics. You can check that out HERE if you need further reassurance
Basically all the standard rental agreement says on light globes is that the landlord has to make sure they are all working when the tenant moves in and the tenant has to make sure they are all working when he or she moves out.
As I said, the only difference I can see is if you needed an electrician to work out how to change the globes.