If you have miles in your existing account which you don't want to lose, simply go online and buy 1,000 miles. Will only cost a few quid, and will keep your account active until your next flight.
I did the same when my dormant VS account was up for expiry - didn't want to lose the 50k miles I had in there, so bought 1,000 online and now have another 3 years of active membership.