Hi Linda
I'm not sure what the status of Grasshoper warbler would be along the Foss near the Nestle factory in York. If you log on to the York Ornithological club web site
http://www.yorkbirding.org.uk and follow the link to "contact us", I'm sure the recorder (Andy) or web master (Ian) would be able to help and would welcome any records for the York recording area. I remember Grasshopper wabler being mentioned at club meetings in the past, in the Fulford area on the wet areas on the banks of the Ouse, but can't remember if anything was every mentioned about the Haxby Road/Nestle area. There is a nature reserve in New Earswick in the Folk Hall area that may hold some warblers but it is a very long time since I went there as it is open to the public only rarely.
Hope you are able to find the information you want. Groper is a nice bird to have on a patch so near to a populated area. I envy you as this is one I have missed this year so far.
Best wishes
annesi