My limited company is in negotiations to sell off one part of its business activities via an asset sale which will include fixed assets, stock and goodwill, from which the goodwill will give rise to a chargeable gain, which could be rolled over subject to meeting the relevant conditions. The same limited company has just completed (within the last few months) the construction of a new building that is both occupied and used for the purpose of the trade (albeit in a different activity from the one being sold). Both goodwill and buildings are deemed business assets, all proceeds have been reinvested.  Are we able to take advantage of the ‘12 months before’ rule to qualify for rollover relief?

Yes you are. See www.hmrc.gov.uk/manuals/cgmanual/CG60620.htm which describes circumstances similar to yours. As you quite rightly indicate, the old asset and the new asset do not need to be used in the same activity - see www.hmrc.gov.uk/manuals/cgmanual/CG60500.htm.