I bought my son a brand new moped in November 2013 it had a few minor problems to start with but all were fixed under the warranty. Back in April the bike seized up completely and again was fixed under the warranty with a complete engine rebuild, The workshop stated that water had got into the oil tank, and tried to blame my son for filling it up in the rain, this was not the case as the moped is kept in the garage when not in use and to fill up the oil. All the same it was fixed under warranty.Four weeks ago the bike was booked in for service after many phone calls and fob offs it was finally ready yesterday (4 Weeks later)My son paid for the service and collected the bike he rode approx. 100 yards down the road and the bike seized up and would not start. He pushed the bike straight back to the workshop. After two phone calls today to see what was happening the mechanic says that the engine has seized up again and that there was about an egg cup full of water in the oil tank again. He says the warranty company won't pay to repair it again claiming that it has been misused and that we have to pay for repairs ourselves. The bike was in full working order when my son took it in for the service he picked up 4 weeks later and now it doesn't work.Where do I stand?I thought I had a contract with the retailer not the warranty company.Who do I go to first? Any advise welcome.Thanks