Aye bendik and IanH,
Not sure what I posted, as I have forgotten the post's title/reference and hence my use of the "Search" window hasn't brought the post up.
However, I would agree that Nigel's suggested tapping/belting areas method, using a hide mallet, nylon hammer, rubber mallet, shot filled rubber mallet, drifts and a 1lb ball pein hammer, does work.
PATIENCE! That is the key, along with copious quantities of acetone, Bulldog BDX release/lubricant and some gentle heat played along the joint from a MAPX blowtorch.
BE VERY CAREFUL AS ACETONE/BULLDOG BDX ARE BOTH VERY FLAMMABLE! 
The cases that I separated hadn't been parted since being assembled by Mototrans in 1974-75, subjected to 1,400 miles in Edinburgh, on salty, wet roads, chucked in a skip, rescued, then sat in a wet shed, supporting a leaking roof, the acidity of the water producing some spectacular efflorescence of aluminium oxide that bonded the cases together.
DO NOT GET YOUR CASES BLASTED WITH ANY TYPE OF "MEDIA", inspite of what might be said, you will spend hours picking the "shrapnel" from all the oilways and flushing them through, with boiling water and "No Nonesense Detergent" from Screwfix, to remove the impacted crud, or pay somebody to do the same, if you trust them?!? It seemed like a good idea at the time!

Vapour blasting! No mention of the glass beads that were added as the vapours were insufficient to remove the aluminium oxide!
Good health, BillR