Take a look at A1 Approved - They offer an administered own pot warranty.
My nett pot (i.e. £150 put aside per car sold less any warranty claims paid out) is now just over £3,000 since starting with them about 3 months ago.
A large proportion of this £3k, if I had used a 3rd party warranty insurance company, would never have been seen again and even then, I was often contributing to repairs when the company either refused to payout or reduced the payout.
It gives you some distance between the customer and you whilst you ultimately can cover what ever you want. They take the hard line, computer says yes or no approach but you can (for the right customer), pay whatever you like to keep the customer happy.