If you don't have the basic Electrical knowledge to fit the inverter, set the parameters on the VFD (Inverters) and wire the controls with appropriate interlocks (or have someone who can to advise you), go the Newton-Tesla route. A little expensive but safe.
Gavin O or E-bay units are often older models of VFD and you may not be able to get the instructions allowing you to set it up (change the parameters). Previously owned usually means limited (sometimes no) gurantee. I don't think the prices are always low either. You need to gave a good "feel for the deal"
Drives-direct do some good "packages" and, judging by their adds in ME/ MEW, have "moved up on the supplier ladder". They certainly advertise assistance and control systems. Again, may not be as low priced as other routes - but you'll have "on-line help".
Finally, the RS route is a good bet - assuming you have the basic Electrical knowledge (or access to someone who does). Bear in mind that you'll need a gew more bits and - as RS supplies to the trade - you'll get lottle back-up and you'll need a "trade account" with them. Maplin don't sell VFDs.
Just remember, if you can blow a single-phase motor it's even easier to blow your 3-phase ditto and a VFD if you don't know what you're doing.