How do I start my raised vegetable plot now?

Bit late but you can still start off most veg they will just be that bit late cropping

Don't agree with Moon grower that it's too late. You could still sow and grow courgettes, oriental greens of all kinds, salad leaves, radishes, or autumn cabbages, baby beet, Swiss Chard, and carrots. If you can get leek plants, you could plant these now.
If you are able to make a deep bed that is boarded, about four feet square, put a deep mulch over the soil about six inches deep and plant any plants through this. It will keep in moisture and enrich the soil for the future. We put a six inch layer of rotted straw straight over weeds (cut down with a mower) in the spring, and things are growing beautifully through this, with few weeds coming through.
Baby beet would be my main recommendation as you can sow a lot in a small space and they are delicious. Have you heard of Mel Smith's 'Square Foot Gardening' book? Or look up square foot gardening on the Internet. It's a great system where you have limited space.

Seeing where Mimzi lives yes you are right Bertie... in the north of Scotland sowing most vegetables would result in a late a reduced crop.

Yes, sorry. It's easy to forget sometimes how much harder it is for those in northern parts with shorter summers. We're so lucky here with summers that last to the end of October and beyond. In East Sussex near the sea, I expect Mimzi has a similar climate.

Having said what I did I've decided we can treat Spinach Beet almost as a catch crop. From sowing to starting picking the leaves is just a few short weeks. Also having emptied the potato bag with the first earlys in it we followed a tip from another GoY'er and put the compost back in, sprinkled carrot seed covered lightly and have now crossed our fingers.

Given we have been getting day time temps. of 26 - 28 C everything is galloping ahead - unfortunately the weeds are too :-(