Mr Spanton - this is my expertise, I make these lenses and fit them for a living. Sounds to me like the 'heights' need looking at. If you take them back to where you got them from, and get them to 'mark them up' using the semi-transparent engravings (you can see them if you hold them up to the light, you'll see the reading addition on the temple side and the lens identifier on the nasel side).
Once these are marked up, the person helping you should then be able to correctly adjust the frame so that the 'fitting cross' is sitting equal to your pupil.
The other thing you should ask for is for them to check or adjust the 'pantascopic tilt' to ensure your getting the best view of the lens.
The other main tip is to try and behave like a cyberman, i.e. keep you head and eye's straight and move you neck if you need to look to the sides, this helps train the eye's to the graduation of the lens.
If you look too far out of centre, you enter a 'soft-focus' area of the lens hence you'll get giddy vision.
Brand's of lenses is an oftern used tool by retailer to part with your cash. Essilor brand of lenses are no better than many other so called 'budget' leneses.
As an example, where I'm currently working and soon to be departing, we use Essilor compatible lenses by a company called BBGR, these are called Selective and Evolis and are very similar to the Essilor Panamic and Comfort range, only a lot cheaper, and you'll be surprised how many people prefer these to Essilor.
Also, we do a very cheap range of lenses by Hoya, called a GP 1.50 - quite a crude design, but very often first time wearers prefer these to the newer supposedly higher spec Essilor designs.
It all comes to down the fitting, and i'm afraid to say that about 90% of the people working in high-street opticians aren't qualified or experienced enough to dispence varifocals. If any of you are buying varifocals, or even hi-index single vision lenses, ALWAYS ask for a DO (Dispensing Optician) to do your fitting and frame choice, these guy's/gals will know what they are doing, unlike an Optical Retailer/Assistant who has only been shown on the job.
Rant over
Any other optical questions, i'll be glad to advise!