Hello Jon,
It is rather telling that despite the fact the company is part of the "Approved Workshop scheme", you do not consider that to be sufficient to instil confidence in the company.
Have you heard something adverse about the company? and consequently are you looking for excuses not to use them? Let me add, it would be wholly inappropriate (and against forum rules) to repeat any adverse unsubstantiated gossip about a company.
I think you will find that even some of the most respected companies will have some customers who are unhappy about some aspect of their work, and they are the ones who are most likely to voice their opinions.
Part of the difficulty of garnering this sort of information from a public forum, is you have no way of verifying the validity of any claims (good or bad) about companies. In customer service circles, it is a well known and recorded fact that an unhappy customer is far more likely to air their views than a happy customer, but also an unhappy customer is more likely to repeat the report several times and to far more people (e.g. friends and relatives) thus giving a wholly negatively biased view of the issue even if its or is not a justifiable position.
In some extreme cases customers have been known to carry out a vendetta against a company and actually spread inaccurate and malicious rumours, that is of course illegal.
If a company has been long established, then they are very likely to be doings something right, but unfortunately it is human nature not to report satisfactory or even good service. It's usually only extremes that get people going mostly bad news, and less frequently exceptionally good news.
And just because one person had a particular issue, does not mean you would experience the same difficulties.
I suggest you take the "Approved Workshop scheme" as something that's far better than nothing. It does at least indicate the workshops have the necessary equipment and hopefully competent personnel, and thus are capable of doing a good job