The first thing infringers do is remove the evidence. Some "consider the matter closed" but this is a mistake - if the infringer does not respect the value of photography then they will search the web and steal from someone else. It is for this reason that infringers must pay in cash.
With copyright infringement there are two payment aspects to consider:-
1. Fee for the infringement, from initial use up to removal
2. Fee for the continuing use once 1. is paid
Item 1 is non-negotiable with respect to how much you charge. Set a fee and stand by it. Infringers will try to negotiate and appeal to your moral side to not pay, or reduce the fee. By stealing your work, infringers have demonstrated they do not have a moral code that matches yours, and should never be negotiated with. Negotiation of fees and terms of business are restricted to those who contact me before using my work.
By offering you a beer and lodgings they have conceded that the infringement will cost them something. Figure out what you will charge and if you are not sure have a look at the London Freelance Fees Guide for online use. Invoice them for at least 12 months use.
As a rule I use this fee guide for any infringement, and then I add 25% - why should infringement be a cost-effective route to a legitimate license?
Good luck. If you need specific help, just PM me.