I'm going to have to reluctantly agree with Liam regarding Muslim communities often being closed up. 40% of my customers are Muslim, and in the end I wouldnt care if they were Satanists, as long as they paid the bill.
I work in South London, and there is a small community in the Streatham/Tooting area; I recently got kicked out of supplying a busy shop in Streatham, the reason stated was that it was because i or the owner of my company are not Muslim. Thankfully i went past there today and the owner called me over, apologised and asked me to supply him again. In East London, we have to employ a Muslim rep, as its the best way to maintain business.
From an accounts/finance/credit point of view, we have been forced to have a policy of looking at the country of origin, before even considering giving credit, sad though it may seem. Everyone from Turkey, Egypt, North Africa and the Kurdish are absolutely fine; some of the nicest chaps you could possibly meet. I often see them smoking outside the shops etc, talking about going to the pub etc; when challenged they reply 'its in my heart'. Unfortunately, my company is let down dramatically by Pakistani customers, Muslim or not; the Turks all tell me its a religious instruction to knock non-Muslim suppliers for money, although i reckon they are pulling my leg.
For the record, English and West African run shops are terrible with money too. Guess that's why we dont run shops any more. Just to add to that, no problems at all with Hindu run shops, Sri Lankans. Best customers by a mile are Sikh.