Mr D suggest you use the largest size of cable the manufacturer of your MCB?RCD says you can fit into the termiantations. Primarily for voltage drop. Conduit is OK but check current UK regs (big assumption, where are you?). Buried cable must be market with a suitable identity tape. This does apply to domestic as well as utility! Liabilty will be all yours for years to come! Is your main dis board protected by an RCD? If it is, you may want to consider one with a lower setting for the garage. The best solution is to have one in the house to provide protection for that cable run. If you can find a source of old wire armour (ask anyone you know who works on the power side of a local utility.
If you want an idea of voltage drop values. Take a quick look at the Martindale site calculators.http://www.martindalecenter.com/Calculators4A_1_SenW.html#ELEC-VOLTDROP or one of the many helpful people who share on this forum has superb detailed view.
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The site is American, but the physics is international. You will have to do some wire size conversion. Martindale will probably have one. A word of warning, still assuming you are in the UK, check your incoming voltage at the consumer unit to take a reference value fr these calculations. For all I know you are at the end of a 0.04 feeder from the substation, already at the lower end of the voltage range for supply. (Soon to be a miserly 207 volts) so your garage may be dim and full of stalling motors.)
Best of luck,