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How much torque are you wanting to put through it?
If it's nothing major then you can get away with a wide jaw, short handle type. Model 4003WT will accommodate up to 38mm: http://www.teng.co.uk/4002wt-4003wt-teng-tools-adjustable-wrench-15-deg-phosphate-finish.html
If you're needing to put more torque through it then you're probably gonna have to shell out a little bit more for something with a 12" handle like the 4005IQ: http://www.teng.co.uk/4002iq-4006iq-teng-tools-adjustable-wrench-phosphate-finish.html
Should point out I'm not sponsored by Teng but I've always got on well with them. Other brands are available, etc.
Need an adjustable spanner that will do at least 36mm - anything better than a Park PAW12 for £25ish?