I would feed some single stranded copper cable up the pipe until it stops and keep prodding it, have somebody in both lofts to listen if the end can be heard tapping anywhere. That should find it.
tallandslim
Thanks for the reply boilerman. The price i've been offered is £300 to £400 less than Vaillant or WB of simlar spec. Just seems such a good deal to ignore.
My current boiler is a Ariston Eurocombi, 3 bed semi and the reason for the upgrade is because we are having an extension built. This will include 2 further bedrooms and a utility room. Thanks.