A very common problem with constructions like this, but nothing to do with the stream - if it was, the original building would have collapsed well before the extension was built. 'Hanging' that modern bit off the rear of the original construction has completely 'destabilised' the original foundations - If the old unit had been left to its own devises, the whole gatehouse would have moved as one unit (as it obviously did until the addition was built) with the only obvious signs being a few minor plaster cracks. Demolish the modern rubbish and underpin the original foundations if required, but the economics are not really viable for restoration unless one puts an extra property on the site or has very deep pockets to extend the place properly, A situation all to common with these old gatehouse properties nowadays, with many also being situated very close to busy major roads. It is also more than likely that local bye-laws will have something to say about any planned redirection of land drainage/streams - they do in England, so there will no doubt be similar rulings in Scotland.
