Appalling response from customer and competitor

It seems you have done nothing wrong. You provided the website, it looks good and you got paid so the customer must have been happy. It terms of the other service providers recommendation to report you to Fair Trading there is nothing to report so don’t worry about it.

I assume you have provided the client with the correct information about SEO and the services you can offer to promote the website. I mean of course your client wants their website to be immediately number 1 on google. But any client that thinks this can easily happen and any service provider saying they are able to do that without a lot of work is just plain dishonest.

Either way the client is within their rights to seek quotes from other service providers for the same service. If they want to accept a quote to totally redesign and rebuild the site on Wordpress without really understanding what it is they want or need than that is that is totally up to them.
Of course it is stupid and ill-informed but ultimately there is not a lot you can do about it. In the end the client will learn a lesson.

The best you can do is provide the client with correct information then it is up to them to decide what they want to do. If they want to continue to use your services fine. Otherwise forget about it and move onto the next job.

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I'm the developer of the site. ...My customer has zero knowledge about technology behind the screen. Why it matters which platform is used to deliver the HTML.? I built the site as requested and got paid for what we agreed with....
The seo company don't know how to use nopcommerce, they want my customer to get rid of me and the site I built entirely....

Hello.  
No worries, actually it's absolutely typical "normal" situation... But U didn't mention who in charge of content/SEO of the site.  So I'd say the SEO on your site is almost/just horrible... 1. no descriptions for any pages 2. Bad H1, H2, and etc. 3. I've visited https://developers.google.com/speed/pagespeed/insights/?hl=eng and checked your site
- First Contentful Paint
4.5 s
- Speed Index
6.6 s
- Largest Contentful Paint
5.9 s.
It should have been fixed before...;).
Best Regards.

I am solicited every day by some wanna be SEO expert about how bad my site is optimized. I own, develop, maintain and do all related functions of running my site(s). I have been a Nop user since version 2.65 on my live sites but played with earlier versions than that, just because it is free its open source and generally it works well. I have used many other platforms over the years for other sites and the SEO crowd will always cry how bad your SEO is, its how they get biz. Convince the not so tech savy that the platform their using is so bad they need to scrap it, but go to a WordPress baffles me, why not Wix and then like you stated your going to need a plugin to handle the ecommerce side of it anyway at the point you state your customer will sell. I would not worry about. I do not know the laws of your land but if he causes you harm by what he is saying, you may have a defamation claim. SEO is a never ending project, it can always be better on all sites. Just because this guy can't use Nop and figure out how to configure SEO without accessing html markup to do it is his issue. Problem I think may be is your customer will need to allow him into the admin to do his SEO work, or the db directly.

I have just read Mortdales string and am shocked to see my email has been modified as this is not what I have written.

Firstly he had no right to publish my email to the client here. Secondly, extra lines and words were added that I never wrote in the email. This is a complete breach of privacy and how dare he share parts of my private communication to the client on this string.

Yes, I was frustrated with the platform as I can see it does not support optimising images. Mortdale did create a good design on this platform. I was never condemning his work. I will be seeking legal advice as my privacy has been breached.