Fire seal between semi detached timber frame houses

I am starting to top up my attic insulation .before i do that I want to fill in a 1/2"-3/4" gap between the roof and the top of the block party wall with an appropriate fire seal. Bought the house recently enough and was told it would be in my best interest to do this. I'm wondering what is the approriate fire seal to use. I see woodies sell a product called (wait for it) 'Fire Seal'. It says on their website it's for boilers stoves, firplaces etc..Just wonderig will it do the job though. Hilti sell a product called CP 620 Expanding fire seal but apart from their website I don't know where to get this. Probably Chadwicks or good builder providers. Does anyone have any handy information about fire seals (price, where to buy, ease of installation) . thanks