VAT flat rate scheme percentage for tyre shop

I would be grateful if anyone would please give me their opinion on the correct FRS % for a retailer of car tyres.

The business sells a mixture of new and second-hand tyres. VAT invoices are received for stock purchases of new tyres, but not of course for second-hand tyres. I assume that, if the FRS is not used, the business could, perhaps, account for VAT on the sales of new tyres in the "normal" way  and the VAT on sales of second-hand tyres under the margin sheme for second-hand goods, but this would necessitate detailed sales and purchase/stock records, which would be difficult to say the least.

The FRS therefore appears to be the best option, but what % please?

Many thanks for any assistance and guidance.