Detail in the link suggests,
"...You can then take your bike in to be fixed, using the voucher towards the cost. You can then claim the cost of the discount (up to £50 per bike) back through the Energy Saving Trust website. The trust will refund the bike owner on behalf of the government on a weekly basis."
That doesn't make sense. You get sent a £50 off' voucher and use the voucher towards the cost of a repair... so the bike shop takes the voucher towards the cost [and reclaims the money from the Energy Saving Trust otherwise they don't get the money?]... but then it says 'you' claim the cost of the discount back from the Energy Saving Trust... so the bike shop charge you full price and you reclaim the £50? ...so why bother giving you a voucher to start with?
signed,
'Confused of Northumberland'.