Alot of the entries for B&Q end 00 or 11. They appear to have direct dialing, like Tesco branches.
11 is store support line with a menu to speak to departments including management.
00 is the main customer number which the 0870 goes to.
10 is fax.
Find the store on the
B&Q website. Look for the fax number. If it ends 10, then change the last 2 digits to 00, you should get the store's general number.
Obviously I can't confirm that this will work 100% of the time but it's worth a try. It's also possible that the fax number may not end 10, but changing the last 2 digits to 00 will work. You will have to try it to find out!