Tiling over outdoor tiles?

Not sure if you're serious or not. Would it not be similar to tiling over a concrete floor?

I was actually responding to 20/20! Maybe with a username like that, they can see a potential issue that we don't ;)

This is absolutely terrible advice

You can't tile over these slabs, get someone that knows what they're at as porcelain can be challenging to work with if not experienced with it. Also, if you're prepared to spend money on porcelain you can well afford to buy a recessed cover, they can be bought for 70 or 80 euro online or at most paving centres

Thanks. I'm not trying to be the devil's advocate here, but might a good base already be laid down under the slabs as they look pretty good from a levelness point of view?

There is absolutely no reason why they can't be laid on the existing paving, it's no different to a hardcore base. It's actually better because it's already compacted and settled. If some are wobbling they can just be cracked with a sledge.

That base is not for tiling , and even if you do tile why porcelain ? It will be dangerously slippy. If you are using tiles make sure they are exterior rated and R 11 rated. The problem with R 11 tiles is the anti slip texture is a bit like sand-paper and dirt sticks to them. I would suggest looking at new paving from patio centers. New paving would be cheaper than tiles and transform that area. Also regarding the manhole covers spend the few quid now hiding them. Galvanised Pavior Manhole Cover 450 x 450 x 80mm - Ray Grahams DIY Store They also sometimes have a 12 inch version for €70. I know they are 3" thick which is perfect for paving, but I have used them for tiling by just filling the lid with 2" sand + cement.

Before I did anything I would take up one of the existing to see how it was laid.

I don't see how you can know that the base is solid without knowing what the base is or how its laid?

Maybe for 5 years its been sitting there unused and if it actually took traffic it would start to move. I wouldn't put down good money onto something that I didn't know & trust.

If you can tell without looking then fair play to you! :)

If it was my money, I would want to confirm how it was laid and see how it was after a few years of use before I dropped a few grand of porcelain onto someone elses work.

If they are done properly and stable 5 years later then I wouldn't have any concerns using it as a base, but who knows, maybe half of them are loose but havent shifted? Maybe they are not actually that level and would cause issues tiling over without a screed on top? It would depend a lot on how much use they have had over the last 5 years also.

Also, Im not talking about taking them all up, just one or two to see whats underneath. That would seem like minimal effort in the scheme of the job for good info in my book.

I would take them up, not least because raising the ground level further with respect to the internal floor level might create problems with penetrating damp.