I will phone in advance each and every time I send as - as has been shown here - the rules change or are confusing and - legal or not - I am happy to go with what the abattoir suggests.
Suziequeue,
Wouldn't worry about phoning each time, the rules are not liklely to change anytime soon, they have been the same since 2007 (England) and 2008 (Wales) and are the same rules in both countries. A slapmark on each shoulder is fine for both the rules and your abattoir, if you want to slap legs as well then no problem.
What is being argued on this forum are technical points between the three countries, and what people do in practice.