arris wrote:yes as mutzy says, most master key systems are on restricted registered keys, so that ya cant just pop down local DIY place an get a copy cut,
most need a offial order, or a certain person with ID to get duplicate keys,
Mutzy is in Australia, you are in England. It's quite different in the US.
Master keyed or not, system uses standard, harder to obtain, practically restricted, civilly restricted or legally restricted.
Standard:
Common Schlage (SC1, SC4) and Kwikset (KW1) are usually single key, but supports master keying.
Harder:
Manufacturer wants to say restricted and won't sell blanks to you unless you have the proper credentials, but they're common enough and there is no current patentable feature, so generic blanks are readily available.
Practically restricted:
Some manufacturers have reserved keyways made in relative small quantity and assigned to specific locksmiths, large companies and institutions. Generic version is typically not available, because the market potential isn't large enough for them to bother making, however they have the every right to make them and it is not against the law to duplicate them using a CNC or an easy-entrie type blank fabrication machine.
Civilly restricted:
Blanks are protected by United States Utility patent and commercial infringement of which may lead to civil actions against the locksmith from the patent holder. Currently, these are Medeco^3, Schlage Everest Primus and such.
Legally restricted:
Keys protected against duplication by federal, state or local
Federal: US Postal Service and some military keys.
State: California, Arizona and a few other states prohibits the duplication of keys suitable for buildings belonging to the state or being used by the state at the time. A locksmith in NV state maybe able to legally an original belonging to the state of CA, but the burden of responsibility is on the owner of duplicate once he returns to CA as the possession of the duplicate is illegal.
Local: Some area such as Los Angeles county ordinance prohibits the duplication of anything that says "don't copy". This is simply the matter of going outside the county boundary to get it copied.
Regardless, many locksmiths will say

"blah blah blah blah blah, it says duplication prohibited, it belongs to the university in town, blah blah blah blah"