Flooding

I suggest you get someone to look at your roof. Check yourself to see if there is an obvious source for the leak (cracks & water ponding on flat roofs, missing / loose / broken slates on pitchec roofs). Also check the attic if you can (can you see direct sunlight throught he roof). It may be that there is only a minor fault / damage (this would explain the long interval between leaks), but he longer the problem is there, the more likely it is to get worse.

BE CAREFUL. DO NOT STAND ON THE ROOF - RISK OF COLLAPSE. USE LADDERS ONLY WHEN ACCOMPANIED.

Oh yes, roof leaks rarely lead to river-type floods (although both result from sudden heavy rainfall), get some buckets and sheets of plastic. Remove anything of value from the room and any rooms under it.

PM me if you want, I'm a quantity surveyor (but I hate my work).

For future reference:

How much water?

Assuming a 'drip' is a 'drop', put a bucket under it and put a second container under the bucket (as an overflow). Check it after an hour and then every few hours to see what the flow is like. If you are up to the task check the outside in the morning.

What type of roof is it (flat / sloping)? Is there separate ceiling immediatly under the roof? (water may have flooded part of the joist space)

How old is the house / extension?

If the extension / repair is relatively new (1 year for a slight leak, 6-12 years for something more major), you might have some comeback on the builder.

The problem with leaks on flat roofs is they tend to originate in one place and drip in a another, by running through the layers of the roof.