.....let me guess.....the ill couldn't turn off the smoke alarm in that house...was that it?...could somebody remember the 'bleeping'...somebody who had been held captive, in that marsworth house? we must have been the surveillance team then, eh?....and that means - MI5 already knows which house would have been the most likely to have been used by their teams...they will have a record of it, somewhere...go figure.