Where can I buy a backpack in Mayfair?

I work in Mayfair and want to nip out and buy a backpack at lunchtime.  Run and Become in Victoria is too far - I need somewhere either in Piccadilly or Oxford Street.

 Does anyone have any suggestions?  I run home from work and am looking for a basic good running backpack.  I can't think of any suitable shop on Oxford St, but there must be one...

 Thanks

Litdog - lots of options:

Cotswold Outdoors at Piccadilly Circus is a good store

Itchy Feet down on Wardour St in Soho (just off Oxford St) is also I'm told very good

There is a cluster of outdoor stores in Covent Garden also

Litdog - just re-read your post.

Reckon Cotswold Outoors would sort you out - obviously it's not specifically a runners' store but it's a big outdoor store and I would expect it to store running packs. It's opposite the Ritz more or less, on Piccadilly.

I tried to find a running pack for to-and-from work purposes a bit back and came up against a volume problem: most of the specialised ones are just too small (like, 5-10ltr). If all you've got in it each day is a shirt, boxers and socks, fine.

I ended up with a 15l Karrimor Silhouette daypack: it has a waist-strap and a chest-strap and does me well.

Adrian thank you, what a good idea, and I know exactly where the shop is.  I'll go there later.

Thanks for the info on packs too - I've been reading the net a lot but got so confused about volume size I thought the best thing was to go and try them on.  15 litre sounds about right - I've got a crappy 12l one at the moment, but the only thing that is good about it is that the size is about right.  I'll look out for the Karrimor one.

Thanks

Berghaus and Deuter seemed to be the other good options at that size.

Some have that airflow webbing that holds the sack away from your back, which lots of rucksacks have now, big and small, but my Karrimor doesn't - it seemed to me all the extra plastic framing on those airflow sacks added too much weight.