UK Map update for TomTom 500

[Ocho]

This may have been done a million times on here, but what's the cheapest way of getting an update onto a TomTom. Do you have to register with TomTom and then subscribe regularly or is there a more straightforward one off way?

Also I need to be able to reuse whatever update I get as my TomTom seems to need resetting every once in a while when it throws a wobbly, hence the need for the update now...

Thanks in advance to all.
Jem