so if its not my tyres, its gonna be someone elses 
I've read posts here from guys who've had a large percentage of their work come from one or both of those jobs and end up in dire.
As Wavie says, it's sensible not to put all your eggs in one basket.
You've got a difficult deciscion to make. Do you go for this account and feel bad if you get it, or do you leave it; only for someone else to get it?
Personally, if it were me, I'd leave it. I've too much work, and only looking to take on better priced stuff; dropping my lower priced jobs, so I can do without any 'moral dilemas'.
But your family comes first, and if you need it, you should quote.
I'm sure this 'old guy' will get by somehow, begging on the street or something; remember people are more generous towards vagrants comming upto Christmas.