Sorry to say 6-8 weeks is the standard Bugaboo time, you would be better off looking on internet or ebay shops for someone who has one in stock, there is a really good ebay seller mcrae3919 or two left feet who have lots of stuff in stock.
have you tried ringing bugaboo, I found them very helpful on the phone tell them what mothercare have said and that you feel this is unaceptable I can try and find a phone number for you if you need it. why is it not under warranty?
well I was suprised that the fleeces were covered by the two year warranty mine had faded on the hood and I rang them and got a replacement free of charge. I would ring Bugaboo direct they are very helpful unfortunately they are crap at answering emails
I contacted them in view of giving my BC an overhaul in time for new bubs and they gave me the details for some megastore - Babyworld in Fulham - that they said is generally in stock with parts. Babyworld 239 Munster rd Fulham, London SW6 6BT Tel. +44 207 386 1904 For what it's worth I got a quick reply from them - 10 hours I think it took.