I don’t know who stocks clothing but the following big retails are all Mammut stockists:-
Blacks Outdoor Division Mansard Close
Westgate
Northampton NN5 5DL
0044 1604 441176
Ellis Brigham Ltd. Queen Street
C/O F.E.B. Ltd., S. Crowther Building
M3 7DQ Salford Manchester
Cotswold Outdoor Ltd Crudwell
Kemble Business Park
Unit 11
Malmesbury, Wiltshire SN16 9SH
And the following on-line retails also have Mammut gear.
http://www.climbers-shop.com
http://www.ultimateoutdoors.co.uk
http://www.tiso.com
http://www.nevisport.com
http://www.highsports.co.uk
http://www.gooutdoors.co.uk
With the Swiss Franc so low at the moment it may be cheaper to get Mammut stuff in Switzerland.
You can find a dealer list here
http://www.mammut.ch/mammut/haendlerliste.asp?action=anzeige&land=14&sid=2