Hi Jack, they are only an hour in front of us so it shouldn't make much difference. Afternoons and weekends will be cheaper bearing in mind that they may be shut today.
You could try Cherrycalls, I haven't used them but know someone who has.
http://www.cherry-call.co.uk/europe/phone_germany.jspIf you use a mobile it is dearer and there is a £5 set up charge but there's no charge, fees or registration required for landline calls and it is tons cheaper as far as I know.