Welcome to Websleuths oldhat and everyone that has joined the past few days.
Going on memory here as I haven't lived in the UK for many years but on terraced houses (row houses or townhomes for Americans) I thought not every house had access to the loft? I also remember fire walls being virtually non-existant and no-one stored anything in the attic. If I am wrong and there is access from all houses Tia could have been moved. I can't get my head around that though.
The water tank - ours was in the airing cupboard and I think this is plausable as where Tia was stored.
Destroyer mentioned underneath the bathtub. Ours was boxed in with plywood and could easily be removed to do repairs. A body definitely could be wedged in there.
The water tank would defo have been in the loft/attic. There is always a brick wall between houses, although not unmovable. The police have admitted their error anyway, so it's unlikely to was moved to next door's attic. Water Tank was mentioned in a local store tonight by gossip mongers, but it does make sense.....unless the body was hidden in a Christmas Tree box, covered in plastic branches and not thoroughly checked. Who knows?