Yes, slapper needs inking - you can do this with an (old!) toothbrush or buy a pad, but you'll need ink - queue "supplies for smallholders" - who'll no doubt post some helpful info.
www.suppliesforsmallholders.co.uk/pig-items-c-131.htmlYes regs say do both shoulders (PRIMO 2007 in England 2008 in Wales) and if you are getting your pig back as two halves, the slapmark will clearly show on the two halves - pigs "dress out" white as part of the de-hairing process. The black is a very thin layer of skin, comes off like tissue paper once scoulded.
Yes abattoirs do make mistakes, but this is generally cock-up over conspiracy, and with food chain it is very unlikely that you will get anything but your own meat back. Yes there are examples before people post, but these are rare.
You should do a reasonable slap - like hitting a tennis ball.
If a butcher is going to cut for you then by all means do as chickenfeed suggests and slapmark the legs as well - that way the prime joints will have part of the slapmark showing. However you do need to have a certain trust in your butcher.
If you plan to try and butcher yourself - unless you are qualified or have practised/been taught and have all the right tools - you'll take at least two days, exhaust yourself, have a complete mess, and probably less useable meat than if the butcher had cheated you of a third.
The best on-line guide to pig butchery is
http://www.qmscotland.co.uk/index.php?option=com_content&view=article&id=112&Itemid=65and select pork from the drop down list - lots of execellant videos
Good luck, and let us know how you get on !!