They have the power to threaten you with arrest, they cannot arrest you.
What they 'threaten' to do, is call the police if you fail to comply with their demands for a fixed penalty.
Now, given that they are paid in relation to how many people they fine, and given that the Police custody suite is in St Asaph, I am sorely tempted to put them to the test.
Wouldn't it be great to tie them up all day in St Asaph, making statements etc.... thus ensuring that they earn a non-viable amount for the entire day.