I just went to the Wolfrace site, typed in DN15 (which is apparently central Scun thorpe) and 3 wheel shops came up.
RICHTOY MOTORSPORT LTD  
2 KINGS STREET 
WINTERTON 
S****HORPE 
DN15 9RN 
01724 735282 
 
GOCAR LTD  
MANNERBERG WAY, BESSEMER WAY JNCTN 
S****HORPE 
DN15 8XF 
01724 855 063 
 1st GEAR CAR PART LTD 
51-53 DONCASTER ROAD, S****HORPE 
DN15 7RG 
01724 874111 
 
All of these people will be able to help you, for your safety please go and visit one with an original wheel bolt and a merc wheel.