Is it safe to install a D LD2 type fire alarm system for three self-contained flats in a Victorian property that lacks a spare fuse in the distribution board for a landlord supply?

Yes it would be safe. So long the panel has its own fused connection unit. It could also be taken from the lighting circuit (Still with an FCU for isolation/protection) that way if there is a problem it should soon be repaired as no one likes to go without lights.

As long as it's not off a pre payment meter it's tolerable.

1 hour ago, AnthonyB said:

As long as it's not off a pre payment meter it's tolerable.

Hello, thank you for responding. I'm uncertain whether the communal lights are connected to a prepay meter. This is the reason why I wish to install a landlord's supply. However, the other two directors of the Freehold company, who are responsible for our Victorian-converted into 4 self-contained flats property, are not willing to cover the cost of upgrading the current distribution board, which would allow us to have our fuse/supply.

I'd make the effort to find out, I've worked on a Tribunal case where a landlord supply was installed when not actually required for EL & alarm and helped the leaseholders win and not be liable for costs.

If it's pre payment only then there are grounds for a landlord supply but these can be very expensive to provide and you are at the mercy of the DNS as to what they will charge for their side of laying in the new supply.