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.