Whether consumable or not I would argue that a spring failing within a week of purchase is not a reasonable time for it to last. The point of SOGA is that the vehicle must be fit for purpose and defect free for a reasonable time. 1 week is not reasonable and the key term is reasonable.
Dealers will consider wheel bearings consumable items as they can fail and will eventually wear out. Fact is that when I bought a 7 year old car, I needed to have a wheel bearing replaced 2 MONTHS after purchase. The dealer refused same as this one saying not covered under warranty as consumable and it it can happen. They had also had an RAC check carried out and a fresh MOT applied. The dealer was forced under SOGA after taking legal advice to have it repaired at a cost to them of £240. I was quite willing to take it further but they realised that, based on the evidence given to them by myself and the finance company acting on my behalf as they agreed with me, it would be best to just get it done.
A puncture and a snapped spring are very different. Whilst springs can just snap, the likelihood is that it was weakened and failing at point of purchase and as such was an inherent fault with the car. It may have passed a visual inspection for MOT and RAC but this does not remove the required liability from the dealer as they would not have put it through any sort of mechanical testing to check how strong it was or looked for weaknesses within the spring that could not be seen during a visual check.
We can argue until we are blue in the face on a forum as the only place to actually check this would be in a legal setting. Whether the OP wants to start down that route remains to be seen but at this point he has nothing to lose by quoting the legislation and sending the letters. As you say the cost is trivial and will the dealer be willing to potentially have to defend a case over £70? He will not be able to claim his costs back except under extreme circumstances where the action is thought to be frivolous and claiming for a failed part on a car purchase a week prior to failure would be unlikely to be thought frivolous.
End of the day you buy from a dealer for piece of mind and to cover yourself from problems like this. You pay a premium for this and in this case the dealer is failing to carry their required levels of care for the consumer.
OP it is up to you. Either pay the £70 and get it sorted and take it as bad luck or try to get it FOC. I will not say it will be an easy road as it will probably takes weeks to get sorted but those are the options you seem to have.