Here is the extract from The Scottish Tartan Authority.

Name of Tartan: Sheffield, City of
Alternative name:
ITI number: 3110
Category: District
Designer / Source: Yellowley, Heather
Date: 2002
Copyright: Slog:
Colour Sequence: RBGKNGNGNGNKGB
Thread Count: R6LB10LG10K8N62G6N6G124N6G6N62K8LG10LB10
Notes:
Sheffield Pipe Band gained permission from the Lord Mayor of Sheffield for a City tartan. Designed by Heather Yellowley of Strathmore Woollens the grey reflects the steel industry, blue and green are the City colours, black is for the coal industry and the red is for the heat - presumably in the steel making process.

Note, that it lists the designer but not that it is Copyrighted.
Contacting Strathmore Woolen Mills would be a good idea.

http://www.strathmorewoollen.co.uk/m...s.php?pageid=2