Hi all,
From "...... Ennerdale Lake in the lake district......", perfect, sorry for some reason I thought you were in Wolverhampton, but the same set up as the Elan Valley (which supplies water to the W. Midlands).
You could try asking the water company what the mean (average), max., min. and "95% confidence interval" for orthophosphate levels are, that will give you PO4 levels and from there you can work out P. They are not legally obliged to tell you, so they may not.
Although P is a macroelement, plants don't use anything like as much of it as N or K, small, dark, often distorted and "purple" new leaves would be the symptoms to look for, rather than the pale leaves and/or yellowing and chlorosis you would get with N, K, Mg or Fe deficiency.
cheers Darrel