It appears that you have lost complete confidence with the Garage and the services provided, which you confirm were substandard.
As discussed you need to write a Letter of complaint to the garage please amend accordingly. Please see:
You could take the car to a different garage, to get a second opinion and costing etc. It will depend on how much work the original garage has already done and whether or not your Van is in a safe condition to drive.
If you get the Van repaired at a new garage, you can try to claim compensation or go to Court for the cost of carrying out extra repairs. You might be able to work with a trade association to get a result (if the garage is a member).
You need to write to the Garage in the above terms and ask them to return the monies overcharged, failing which you will take them to the Small Claims Court. This letter is called a "Letter before Action" and the link is outlined above. You must, of course, prove your losses so keep evidence such as receipts and emails etc.
As your claim is worth less than £10,000, then it must be issued in the Small Claims.
To Issue at Court, you need to complete a Form N1 together with the appropriate Court Fee. The Court Fee would be based on the Value of the Claim.
There is nothing stopping you from Issuing the Claim yourself.
I also attach herewith a link that will assist you in issuing the claim should you wish to do this yourself:-
As stated, you can issue the claim either in a paper and at your local court or you can do the claim online via MCOL, the attached link above will explain how it all works.
Finally, you may want to contact the Trading Standards for the poor service to Customers, you can do this via the www.citizensadvice.org.uk/consumer/get-more-help/report-to-trading-standards/
I wish you all the Best.