We've been CHAS for a few years. Also Constructionline and SMAS.
I wouldn't recommend any of them if being done solely to get extra work. We have had nothing through any of them, directly at least.
Indirectly, a lot of large firms insist on at least one of these anyway, and you have to pay for the privilage regardless or you're out. For this reason alone for us, it makes sense.
One other plus point, if you're ropey on your H & S then you'll find any of these schemes have people willing to help and advise. You'll end up being more aware if nothing else.