In having the bonfires you have, effectively, sterilised the soil. You will now need to add a lot of humus rich organic matter such as garden compost, spent mushroom compost or well rotted manure to the soil and keep digging this in. Grow something like a green manure crop and dig it and repeat if needed. Eventually the worms will reappear but it could be a while. Incidentally if the old sheds you burnt were painted or had sealant on them this too will have got into the soil.

welcome to GoY.
The ash will not do any harm but as Moongrower says any paint etc may do. the heat will have driven the worms away. they will come back in time but improve the soil while you wait.

Save all of your fruit & vegetable peels/scraps from the house, cut them into small pieces. Spread them over the soil & work them in. Ground up egg shells are good too. Add some old leaves & grass clippings then wet everything over real good unless you get some rain. You now have a free-for-all buffet for the worms & microbes. Keep the ground moist. The worms will move in very quickly & go to work.

Thank you all for your comments and advice. I am a novice gardener so please forgive me if I seem dense. Moongrowers advice was very helpful and has raised a interesting point regarding burning old sheds treated with wood perserver creosote and paint etc. Would this this be toxic to any vegetables that I want to grow this year,or should I wait until next year. Iam looking to grow tomatoes, Califlowers, beetroot and leeks.

Well the toxicity is in the soil Andy, as to whether it can then be taken up into the veg. you plan to grow neither Bulbaholic or myself are sure... neither do I know how long the toxins will remain in the soil. My only suggestion is to try growing just a few veg. and see what happens, but don't plant anything until you've added humus rich organic matter, this may mean you are better to wait until next year to plant veg. Incidentally where are you in the UK, this will affect whether you can grow tomatoes outdoors or not.

Many thanks again I was thinking about adding well rotten horse manure as there is abundance of this for free in our area (Mid Suffolk) very mild weather.Also if Iam not growing veg this year can you think of any to plant to improve the soil

Well rotted horse manure would be excellent. Try a green manure crop possibly phacelia, check out the link below

https://www.greenmanure.co.uk/products/phacelia-green-manure-seeds?variant=1182263180