I have been learning Dutch now for 3 years seriously. I have learnt via the Dutch Language School in Oxford. There courses are great and this summer I am doing my Dutch GCSE exams. It's hard work but well worth it. The courses are not cheap but if you work out what you get not that expensive either. I think the beginners was about £200 for a one year course and there is 25 lessons in the course but you learn so much more than just the language. There website is http://www.dutchlanguageschool.co.uk
. They are based in Oxford but do correspondence courses but if you live within easy reach of Oxford that have classes. Karin is an excellent teacher. If you contact them do tell her I told you about them. Also I have another teacher in holland in Grongingham and she teaches via email and via skype. I have had just 5 lessons of 20 mins and my Dutch has improved so much. check out http://www.sayitindutch.com
I am doing her digidutch course now and it's cheap and brilliant and you can do a sample lesson before you sign up for the 6 week or 12 week course.
Hope this is helpful.