According to this excellent article on the 407677 the value of R15 is 100k 0.25W...
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
Brown, Black, Black, Orange on the one I have in front of me. I think its possibly a bleed resistor to help the gas solenoid to close quickly - back EMF etc. but that's just a guess - if so the boiler will work quite well without it.
Good luck...
Peccavi, May 2009
