Successful northern strawberry varieties please

[NorthernJoy]

Hello
Having lived in the Borders and now in north Northumberland I've never managed to successfully grow strawberries despite trying several popular varieties. No matter how I grow them I only ever get a few poor ones per plant and it's not been worth the effort at all. However, I know that up here esp. in Scotland strawberries can grow very well outdoors as the pick your own farms testify - but what is the elusive variety I need to grow for a substantial, worthwhile crop? Farm growers will not tell me what they grow and I don't know anyone who grows them locally to ask.
If you have successfully grown strawberries up here then please would you tell me which variety you grow? I want to keep trying, but if I can't make a success of it in 2015 I'm giving up on them and sticking to other fruits instead.
I need to know please:
1. The best northern variety for a heavy crop?
2. How many plants I'll need to give me at least one punnet per week in the season?
3. Whether or not it's worth raising strawberry plants from seed in the north, or should I keep relying on buying bare roots?
Thank you.

[Alison]

How are you growing them? In the ground or pots? I find a much better harvest in the ground but your soil will also make a difference. If you've tried a few varieties them I'm afraid it could be the conditions not the variety, perhaps worth thinking where you're planting (are they in sun?), spacing etc. Re varieties, probably best to ask around amongst your neighbours are what works for them. Could well get some free runners off them too

[Bigmallly]

Because of its tolerance to winter frost Sonata is very much suited for outdoor production in more northern and colder areas.

http://www.flevoplant.nl/de/pdf_rassen/sonata-eng.pdf