I'm afraid your supplier is correct, you'll get a very uneven hedge which will take quite a bit of management to look right.
Have you thought about planting a Thuja hedge rather than Leylandii - a thick evergreen hedge, which tend to fill out at the bottom of the hedge better than Leylandii, are much more interesting to look at and with a lovely smell of cedarwood (it's a type of red cedar) and with none of the drawbacks of Leylandii - for instance, you can cut right back into the old wood and it will regenerate and grow green again, so it's not a disaster if you can't get around to trimming it one year, whereas if you do that with Leylandii it will stay brown for ever. And I don't think the Thuja suffer from aphids as the Leylandii do either.
Or there are lots more choices of hedging
http://www.hedgesdirect.co.uk/acatalog/western_red_cedar.html