Let us assume that you are adding new staff with similar skills to those you already have, in similar posts.
The first thing to do is get a job description together. If your existing staff don't have them, that's illegal under HR law (you wouldn't be the first SME business not to have employment contracts/JD's). I can recommend a great HR lawyer who can get that done in a day, your problem then will be negotiating with your staff for all the unofficial stuff you have had them doing!
JobCentre+ can help you create a JD for a new post, but they will then want to advertise the job. Honestly, much as though they have great backend staff dealing with employers, their front end staff leave a lot to be desired/don't care about your brief! The front end are simply targeted with getting people off Job Seekers allowance, and if the candidate express a slightest bit of interest they will send them over
Most recruitment agencies can also help you create a JD, but expect to pay for this service, either as a lump sum up front or an extended percentage fee.
Assuming that this is for extra staff under existing posts with JD's, go on to one of the big jobs boards and tap in the top three skills under a search. How many people come back nationally with those skills? Less than 50 and you have a problem, and are best using an agency. More than 1000 and you can stick an advert in the local newspaper, or use JC+
In between - depends how much of an HR department/process you have, and whether they are busy on existing issues. Most HR departments love getting away from the grind of people missing work, falling over, injuring themselves with pencils, etc. However, they have limited skills in finding staff, particularly in specialist areas where most of the ideal candidates will be off the market/hard to find.
I would also recommend you ask your existing operations staff for recommendations, and also bonus them for always be looking around for great talent. They will always be your best source of staff, as they will have checked out their technical skills and social fit before even thinking of possibly recommending them.
The other issue you have as a small company is attracting talent, and particuarly so in these dark economic climates. Most of the well trained people will come from corporate backgrounds, and jumping to a small company from a safe "with BMW and pension" corporate will be a hard sell. Agencies are used to this problem, and should sell this for you.
If you choose an agency, choose a specialist - three sectors at maximum, not someone who started out doing secretaries and now does head hunting! Most agencies will be flexible on fee's at present, expect to pay 15% upwards for standard skills and 30% upwards for specialist skills - could be 200% for recent technology skills. I would avoid the high street brands, as they charge high, and most are a Walls prime pork sausage machine as opposed to JC+'s Tesco own brand.
If you need any help, drop me a message.