The simplest and most efficient way of catching Grey Wagtails at Heysham is to create a 'corridor' pointing north-westerly in an otherwise fairly enclosed mist net ride (or whichever degree of northerly is the favoured direction of autumnal migrants at your site) and place a CLEAR tape (Roche works very well) under or behind the net. Try and make sure that the location does not have an easy perch behind the net (a nice dense bush without obvious open branches is best - also giving the net shelter/background), or the birds may fly beyond and see/then avoid the net
There is absolutely no need to "make the site open" for an "open area-loving" species. Grey Wagtails dont mind enclosure at all, indeed quite the opposite e.g. along the gloomy 'gorge' section of the Hindburn at Millhouses, therefore it is not a problem bringing them into a relatively enclosed area where your mist net has the necessary 'background'. Creating the corridor along the natural flightline of the autumnal migrants is a similar principle to landing an aircraft!