Limited Parking? what does it mean?

Yes, two of the Hotel Hoppa buses still stop there,

There is also a local bus route #423 which stops almost straight outside, and takes about 20 to 25 minutes less than the Hotel Hoppa, and is cheaper at the London flat rate of £1.75. Faster as it's not going to every possible hotel on the way.

You can just book a PI room for yourself.

Thanks very much for the quick reply.

If the Premier Inn has first come, first served parking then they don’t take payment upfront. You pay for parking at check in if they have a space available.

Premier Inn don't charge up front for their parking as it is in short supply - if you have paid for it already that suggests you have purchased from their partner, Holiday Extras

You can take the Hoppa H5B or H5C or H5E - they all take 30 to 35 minutes so doesn't really matter which one you take

Why would you book for two if there is only one of you? If it's just you then just put your name on the registration form.

Thanks for all the replies. I stayed at the M4/Junction 4 PI about 10 years ago and I thought I'd paid for parking in advance, obviously my memory isn't good.
On the point about putting 2 people down, as my wife would be using the room for a few hours before her flight, I wasn't sure if they'd need to know.
Thanks again to all of you.