You could replace the two dishes with the one dish, that way you know that the LNB would definately fit
Although it is possible to fit a dish without a sat finder (like what you linked to on the maplins site) it is very hard and very hit/miss.. I used to do it with someone watchign the TV with a handheld radio to give me a running commentary as I moved the dish about
The sat finder though is great as you can fine tune the orientation as just a mm can make a hell of a difference.
Try eBay as that's where I got my last sat finder for less than half the price delivered than the unit you linked to.