Hi Richard
Referrals, in terms of cost per enquiry/job is best, just need to be sure you know where they got your number from, not just that they are referred, this is so you can truly see how much work has come from each of your marketing efforts. If you know it is a referral that's great, but unless you also know they got the number from the leaflet that has just dropped throught the door and reminded them your reports are not accurate. This also relates to repeat clients, by asking where they got the number this time you can see that they got your number from your mailshot or email, so can measure how effective that campaign has been.
Without a doubt next is Database marketing to very targeted niches. Constant contact with existing clients either through email, mailshots, postcards, telesales, the existing client goldmine.
One idea would be to set up filters to find clients in targeted areas on a run back to the workshop from various 'remote' areas you may work in. Then when you are working in a remote area you can email or mailshot all the clients in that filter and say you';re working in the area on XYZ, and make an offer. You'll usually get a call from someone on the list to save you travel costs OR as we use it, to allow us to book jobs close together and fit an extra job in the end of the day. It is not unusual using this method to be working just 10 minutes drive from the last job. This gets the 'per van average' up. The great thing about using email is you can include links to specific landing pages on your website with offers on, and this also allows you to measure what is working and what people are looking at, making future marketing more effective too. If you can integrate this with something like Mailchimp you can also see exactly who is opening your emails, and even which links they are clicking on.
Put the blanket marketing shot gun away, get the rifle out, and put some laser sights on it.
Internet / Web next most effective, in terms of cost per enquiry and cost per job. This is a mix of Google Ad's / SEO. Website is integral to existing client emails too, as above.
Leaflets to Targeted areas next best.
Simon said something similar above, basically you need dozens of little things generating one job a month cheaply.
Al.