I haven't used Trustmark but seen a couple of positive reviews for builders locally.
We actually offer to pay builders by the day, then pay their accounts at the builders merchants for the materials , rather than ask for a quote. This has worked for us though perhaps its unusual?
This is probably only something you would be comfortable with with a trusted builder (with at least good word of mouth recommendations), and also perhaps if you are around during the day so you know they aren't taking the mickey, and understand a bit about what's required (sounds like the latter at least applies to you). I don't honestly think most of them would take the mickey, as they a re generally so pressed for time as you have discovered.
WE are lucky as we have lived here for a while and had a lot done, mainly by just a couple of builders one of whom is really a friend as well and who we do trust (and he is very sought after and all his customers pay him by the day!)
I like the feeling of transparency about exactly what is being done, and for a hard-working builder it reduces the risk, the up-front 'lost' time devising a quote which then exposes them if the job turns out more complicated than anyone could have anticipated. (our hosue is very old, so it always turns out more complicated than anyone could have anticipated.) The one time we accepted a quote from a tradesperson recently, he realised about 1/3 of the way in he had bitten off more than he could chew and basically abandoned the job - taking no money from us, but leaving everyone fed up!
If you are paying a day rate , if something does turn out to be more tricky, or you a re wanting something done to a higher standards or a more expensive material than they had anticipated you are in a much stronger position to make your decision and pay accordingly, rather than have the builder hurry as fast/cheaply as poss to maximise their profit out of a fixed price.
That doesn't mean you shouldn't get the builder to talk you though how long they think it will take, in detail - but they are not at risk the same way. And you know, just occasionally, things go quicker than the builder anticipated, which tends to make them just as happy as it makes us, and they really are not bothered to have a day less work. They usually gleefully tell you how they can have some beers tonight and a lie-in as they don't have to come back tomorrow after all :)
The only time I have felt a bit 'ripped off' when doing this, is a couple of times when the builder has made a mistake and had to go back over something and we have still been charged - but it occurs to me as I write this that this is something I could try to have an agreement about beforehand.
I'm genuinely interested to know what other people think about this approach - we've found it to be useful with several small builders over the years.