SEcond attempt to post. Just got it finished an hour or so ago and our internet went down.
Guard was cut and shut to match wheel. Trick to cutting curves on guards etc is to use 'worn out' 1mm angle grinder blades ie ones with 15mm of blade left as their small diameter allows you to cut curves All my worn out blades are kept on a nail by my vice for this purpose.
How long to make a guard. Some authorities seem to want the guard to vertically cover the wheel, so I suggest this method. Keep the guard a bit shorter for looks, but overhing the number plate. If they really get tough, cut uo a piece of 8mm rubber 6" wide and bolt that in a curve like a 'mud flap'
Others require the guard to come to 45 degrees past the axle. I use this triangle made from a piece of harboard to quickly ascertain the required position. 'Scientific' and can ba argued as such ie a definite measurement as listed in regs.