A single slate is often straightforward
A slate may slip because the nail has failed, the slate has split around its fixing, or something has struck it. If the surrounding roof is sound, the repair can be local. The replacement should sit in the same plane as the neighbouring slates and maintain the proper headlap, rather than being wedged in with a blob of sealant.
Older roofs can be less simple. Nails may corrode over a wide area. If several slates have already been re-fixed with hooks or clips, another slip may be part of a broader fixing problem rather than bad luck.
Look at the rows around the gap
From the ground, compare the courses on either side. A slate sitting low, cocked at one corner or exposing more of the slate below can allow wind-driven rain through. The underlay may catch some water, but old felt can be brittle or torn and should not be treated as the main waterproof layer.
Stone-built and Victorian terraced housing forms part of Blackburn's older housing character, and slate roofs are common on buildings of that period. Roofs that have been repaired many times can contain a mixture of original slates, replacements and different fixing methods.
Matching the replacement
Thickness and size matter more than colour alone. A slate that is too thick can lift the course above. One that is too short loses overlap. Artificial and natural slates are fixed differently, so a roofer needs to know what is already there.
If a matching reclaimed slate is available and suitable, it can sometimes sit better in an old roof than a visibly different new one. The choice depends on the existing covering and condition.
When several have moved
After strong wind, look from ground level at both front and rear slopes if you can. Slates may land in gutters or the garden without leaving a dramatic hole when viewed from below.
Several slipped slates in separate parts of a roof justify checking the fixings more widely. That does not automatically turn a repair into a re-roof. It does mean the cause of the repeated movement should be understood before paying for the same patch again.