A You will have to take into account your staff's level of knowledge, many of whom, I'm sure, will have a good basic level already. The Wine & Spirit Education Trust's qualifications cater for beginners right through to more experienced retailers. They also run a range of evening and one-off courses and can tailor training programmes for individual courses (wset.co.uk).
An alternative would be evening courses run by your local authority (check its website or your local library). Larger wine suppliers are often happy to arrange training for your staff on their specific products.
If you're planning to expand your beer range, consider courses by the Beer Academy (beeracademy.co.uk) or Brewing Research International (brewingresearch.co.uk).
As a cheaper alternative, why not use the expertise of your more knowledgeable staff, and pay them extra to tutor newer staff. In addition, encourage staff to read widely - why not keep a mini-library stocked with bargain books from a web retailer? And, of course, magazines such as Off Licence News and Wine & Spirit are a mine of useful information.