Your car has no spare wheel. Here is what that actually means
Open the boot floor of most cars sold in the UK in the last decade and there is no spare wheel under it. There is either a space-saver, or a canister of sealant and a small compressor, or occasionally nothing at all.
This was a weight and packaging decision, and on the whole a defensible one. But it means the thing you would have done in 1998 - fit the spare and carry on - is no longer available, and it is worth knowing that before the night it matters rather than during it.
What a sealant kit can and cannot do
A sealant kit injects a liquid compound through the valve and then inflates the tyre with the compressor. As the wheel turns, the compound is thrown outward and plugs the hole from the inside.
Within its limits it works. Those limits are: a small puncture, in the tread, that has not yet been driven on flat. A sealant kit will not seal a sidewall, will not seal the shoulder, will not close a split or a tear, and will not do anything useful for a hole much over a few millimetres. It cannot help a tyre that has already been run flat, because by then the casing itself is destroyed.
Two practical points. Sealant canisters have an expiry date, typically four to five years, and almost nobody checks theirs - go and look at yours now. And using sealant usually means the tyre pressure sensor inside the wheel has to be cleaned or replaced afterwards, which is a cost people are not expecting.
What a space-saver is for
A space-saver is a narrow temporary wheel, normally speed limited to 50mph, with a sticker on it saying so. It is designed to get the car to a fitter, not to complete a journey.
Because it is a different width and often a different rolling radius from the road wheels, it affects braking balance and stability, and on a four-wheel-drive vehicle it can load the centre differential continuously. It should go on a non-driven axle where possible, and it should not be left on for weeks - which, in practice, is exactly what happens.
Space-savers also lose pressure sitting in a boot for years. A flat spare discovered at the roadside is a particularly demoralising moment.
At the roadside
If you are on a motorway or fast dual carriageway, get as far left as you can - the hard shoulder, or an emergency refuge area - and put the hazards on. Get out through the nearside doors, get behind the barrier and up the bank, and stay there. Do not sit in the car and do not stand on the hard shoulder.
On a smart motorway with no hard shoulder, if you cannot reach a refuge area, National Highways advises staying belted in with hazards on and calling 999.
Before you call anyone, find your location. The blue and white marker posts along the verge every hundred metres, and the driver location signs, give an exact position. That single piece of information shortens an arrival time more than anything else you can say.
Why mobile fitting exists
The absence of a spare wheel is the reason mobile tyre fitting stopped being a convenience and became the default answer. If the car cannot be made drivable at the roadside, the alternatives are recovery to a garage - expensive, slow, and only open certain hours - or waiting.
A mobile fitter brings the replacement tyre, fits it, balances it and torques it where the car already is. For a driver with no spare and no realistic way to move the vehicle, that is not a premium service. It is the only service.