Power Usage: anywhere from 50Wh to 200Wh.
Solar Panel: 40w
Daily insolation: from 2 to 4(I live in Scotland)
preferrably solar only.
OK, let's work through it a little.
A 40W panel in the real world will give you about 2.5A into a 12V battery.
2.5A x 3hours = 7.5Ah per day
7.5Ah x 12V = 90Wh per day
There are losses involved in putting power into the battery and taking it out again (a useful analogy is a water tank with a slow leak) - I usually count it as 10%.
90Wh x 90% = 80Wh (approximately).
So with a 40W panel you'll need to use less than 80Wh per day, on average. If you're using an inverter you'll have another 15% (or thereabouts) loss in the 12VDC->230VAC conversion, so you'd only be able to use 60Wh.
The usual rule of thumb is to size your battery at the C/20 rate (meaning Capacity / 20 Hours), which in this case means you multiply 2.5A by 20 hours, and get 50Ah as the right battery size. That said, I've never had any trouble charging my 1000Ah battery bank with a 20A solar array, which would be the C/50 rate.
So I'd suggest a 70Ah flooded-cell battery. Given your short daylight hours if you keep a close eye on the water level you might not even need a regulator. They're pretty cheap to build though, if you're a very DIY kind of guy:
http://www.homepower.com/files/hp70-40.pdf
You can find the MOSFET and Schottkey diode for it in an old PC power supply. Here's a PCB I've designed for the circuit:
http://www.otherpower.com/images/scimages/236/switchreg.PNG
BTH