Of course it would be a good idea. Since your diagnostics is £15, and if you think you must have a price on your site, then simply state "Starting at £15". On my site, there are only 2 places you can find the prices - and that was simply for transparency and the fact that I am 1 of 2 shops in the city. One is on my Managed Services page for obvious reasons and the other is deep within my terms and conditions. Though this will all change soon as I am almost done with my new design.
As for your £15 diagnostic, if it takes you less than 30 minutes to find the issue, then that may be reasonable considering the competition. Any longer and your losing money IMO.
One thing to note that no one has asked is that with the competition around you, are they licensed? Insured? I dont know about the UK, but here is states you have to have a business license and certificate of occupancy if you have a retail location or a DBA if you have a home business. And to charge sales tax (VAT in your case I believe) you have to have a Sales and Use Tax license. Since I have a home business, I have a DBA, Sales and Use Tax license and I am also insured.
Check my
managed services page and also
@Barcelona web
site and
@McFarland IT web
site to get a rough idea.