[eye30]

The rubber pad insert on the flip key had been breaking up for sometime and being lazy had just used clear tape to hold the pads in place.

Anyway it came to a point of no return at the weekend as two of the pads had given up working, the unlock door and lock door.

Looking at options there seemed to be  three.

Option 1 was a new key including the inners and it would require progaming to the car. Expensive option I think £90 plus.

Option 2 was an empty new fob case. Reasonable cost, £5 to £10. Required you to open current fob case and transfer inners to new case but you could damage the board and transponder

Option 3 was to buy the rubber insert. Very cheap at £3.99 for 2.

No need to open the fob case. Just pull the old rubber out and replace with the new.

I went for option 3.

Just word of warning. Don't remove the plastic insert under the rubber insert as it is a sod to push the 4 pegs back into its holes as the thickness of the new rubber makes it hard to reseat.

I reseated the plastic insert then pushed the rubber into the side gap.

Photo's, hopefully in order, showing how I replaced rubber insert.

[Greyhound]

Are you saying it is best to dig out the 3 rubber inserts but leave the old plastic 3-hole frame in the key, then push in the 3 new rubber inserts?

Can you say where you found the inserts? Assume online somewhere.

[eye30]

Yes, just pull all the rubber away from the key and leave frame in place.

Only 1 insert with 3 protruding stalks which fit into each button hole to activate the desired action, open, boot or lock function.

If you look at the picture of the reverse side of the rubber, there is a very very thin lip which pulls over the frame.

But as I found the rubber prevented me from reseating the frame with the rubber in place.

So I reseated the frame then set the rubber on top.
Pushed along sides with a very very thin flat head screwdriver provided with the inserts or even your thumb nail. Tight fit so happy with end product

Bought off Amazon.

[The Gonz]

Hmmm... looking like a 3D printing opportunity to me, but eBay / Amazon is definitely the way to get back to original state.
I've already had success printing a plastic-to-replace-rubber solution for the boot release 

STL file Kia Rio Boot Release Switch Repair 👢 ・Design to download and 3D print・Cults

[Diviner]

Thanks for sharing, eye30. 

Facing two problems on both originally supplied fobs, I unsuccessfully attempted your Option 2:  swapping to a new plastic housing. 

In both cases the rubber buttons had deteriorated badly after only 2 or 3 years.  I had then used Shoe Goo to seal up the rubber, as I was concerned about water ingress.  (It sets quite firm, so it's very robust, but the buttons are not functional.) 
Someone had claimed the damage to the rubber was due to "fingernails".  However, mine are short.  And on the second fob, as a precaution, I almost never pressed the buttons — yet the rubber still deteriorated quickly just from the key being in my pocket and being gently handled. 

I suppose I'd dropped the keys once or twice, and the key blade in each started to come loose as the socket holding the shaft had developed a crack in each fob. 

I purchased replacement plastic cases, complete with blank key blades, cheaply online.  Then I found someone here to perform the key cutting (discounted slightly for doing two).  But I couldn't manage to remove the transponder.  I tried prying it with moderate strength, soaking in nail polish remover (~70% acetone), and soaking in pure acetone.  I wasn't willing to try using full strength, for fear of snapping the transponder.  Nor did I try cutting up the original plastic housings. 
So in the end I still have the Shoe-Goo-covered buttons, but swapped the new spring, socket & key blade into the old plastic housings.  (Unlike the replacement housings that come with screws to hold the back in place, I had to dab dots of superglue to hold the back in place.) 

I agree that your Option 1 (brand new key) is overly expensive.