| In our case all I did was reseat the connector under the passenger seat and then tape it with insulating tape to make it less likely to slip. It only takes a tiny flex in the contact to trigger the light, then it stays on until reset. Once taped the light stayed off until we sold the car 18 months later.
With the tool, resetting the light takes about 10 minutes the first time you do it.
If that section of the loom is damaged then replacing it will take a decent mechanic a short amount of time as it's just run under the carpet into the bulkhead (IIRC), but you'll still need the reset tool. Obviously at main dealer rates that's still a lot of money - I pay my local independant about half what Mercedes want per hour to service my other car and the mechanics are all Mercedes trained and certified!
IIRC the tool I bought was described as "BMW AIRBAG SRS FAULT CODE SCANNER/RESET TOOL"
Similar compatible ones were "BMW AIRBAG RESET TOOL OBD2 & 20 PIN" on eBay at the time (the seller confirmed its suitability for a MINI)
If you PM me an email address I can send you photos of the tool and connectors it has but you off checking the connector on the car first as I can't remember which connector it has - round or rectangular.  | 
