Rick is correct, behind glove box, which you remove, then air ducting tube and it slides in almost behind central consul. Filters are £6 on line at Vauxhall-car-parts.
To get to spare wheel lift up carpet in boot, you have to prize up the two metal clips, this reveals metal lidded compartment, remove lid and in there should be jack and bits. Also located here is bolt head for spare wheel carrier.
In most cases you will find the nut holding the carrier will be seized and will be a nightmare to remove. It can be nigh on impossible to remove and with luck will shear off.
Additional point. Look under rear of car and your spare carrier may have a securtity clip which needs to be undone. if this was not enough you might also find that the wheel centre has a plastic washer with a bolt through it which is accessed under the rear seats!!!! i assume this is a security feature to prevent the spare being stolen. I doubt you will replace it as it is an absolute sod to re fit.
If you have a momment suggest you have a very good look at your leisure, its not the thing you want to find out on a cold wet evening!!
Good luck
D