Wait for a dry breezy day ( think you're in the west of Scotland like me so you might have to wait a while for a dry day ). Wash the blanket in cool water with a little mild detergent in the bath, handling it gently. Rinse out well, squeeze out as much water as you can without being too rough then hang it out on the line to dry.
The last part will be the tricky bit as it will be very heavy and drippy. I usually just bundle it up and run as fast as I can for the door - it's not very effective but it amuses the cats. Then I put on dry clothes and wipe up the puddles.