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Basic Recipe - make sure you use a live culture yogurt as a base

Ingredients:

1 quart (32 ounces) of full fat sheep or cow's milk (pasteurized)
2 tablespoons of previously homemade yogurt or plain unflavored yogurt with active live cultures
2 tablespoons of full fat milk (same type)
Preparation:

Start with all ingredients at room temperature.

Heat the milk just to the boiling point and pour into a non-metal container.
Let cool to lukewarm (100-105F). A skin will form on top.
Mix the 2 tablespoons of yogurt (homemade or commercial) with 2 tablespoons of milk.
Add to the lukewarm mixture, carefully pouring down the side so that any skin that may have formed on top is not disturbed.
Cover with a clean dishtowel and place on another towel in a warm, dry place for at least 8 hours (or overnight) until it thickens.
Note: 8 to 12 hours is best. The longer the yogurt coagulates beyond that time, the more sour the taste becomes.
Carefully drain any excess liquid.
Refrigerate for 4 hours before using.
Store in the refrigerator and use within 4-5 days.
Don't forget to save a small amount to make the next batch!
The yogurt can be eaten as is, along with the creamy skin on top.

To make the thick yogurt used in many Greek recipes, follow these directions after step 6.

I think the yogurt might be strained in a cloth to thicken it but I am not sure.

Regards a live culture , there is probably a shop near you in Birmingham that sells a live yoghurt . 
I know I buy live yogurt quite a lot from the Asian shops ( Indian /Pakistani/Bengali ) I would think it is much the same type of culture as used in the Greek yogurt .

Remember , when you are making this yogurt that many of the commercially made brands have a thickening agent added to them so you might not be able to achieve the exact texture as that you buy in a pot at the shops .

Thank you guys for your replies and apologies for being slow to reply myself.

I bought an Easi-yo kit from Lakeland which was quite frankly awful to taste. I found I couldn't ignore the taste of the milk powder which is the base for the powdered mix. Thankfully Lakeland have a strong returns policy like nowhere else, so I returned it and bought the yoghurt maker suggested by Olivia. It certainly works well and I'm just about to put my 3rd mix on overnight ready for tomorrow.

My second attempt tasted alright but was quite lumpy as though curdled. Not sure why that might be, but what I did was leave some yoghurt in the bottom of the pot then added the  1.5 pints of milk to it. I suspect the yoghurt didn't properly blend with the milk. Tonight, I'm taking care to mix it in a little bit of milk before pouring it into the milk. Hopefully the bugs will be happier.

I must investigate my local Asian shops for live cultures sometime. So far I've just used a small pot of Yeoman's plain live yoghurt as my first starter and that seems to work well.

There's quite a bit of whey being produced which I just pour out and throw away. Apart from using as a preserver for Mayonaisse, does it have any other uses ?

I'm definitely going to try using whey in my pizza dough this weekend I also like the idea of using it as a marinade as well.

My 3rd batch of yoghurt this morning went through straining and boy what a result ! I'm going to be trying so hard not to gulp the end product down all at once. Lovely and creamy, though I did detect a slight hint of cheesiness in the taste which made me think of what you said about making cream cheese out of it.