My Panda 4x4 is coming up for 3 yrs old. I am considering upgrading to a 500x.

Not too bothered by spec apart from needing auto, 4x4 and 17" wheels.

What sort of discount should I be expecting ?

I have no idea what discounts are available, but there should be some very decent, low mileage, almost new or ex demonstrators knocking about.

I have an auto 4x4 and apart from some teething problems, all now resolved, I'm loving it, despite its 18 " wheels

Thinking of a new one really...........

I've heard bad things about Arnold Clark, but have no personal experience of them. Is it worth getting a quote from Carwow? Again, never used them but it may be worth a try.

Carwow - first two dealers - first £1000 off and second £2000 off.

Wow indeed - best offer is 19k on a rrp of 23.5k

If my panda 4x4 is not in negative equity based on their trade in, I will go for it !!

That sounds a worthwhile discount. I'm sure you've weighed up the pros and cons of new vs used, but I paid 12k for my 17 month old 15000 mile 500X which has a new list price of just over 20k. That suggests a depreciation of about 10k on the trade value.
Of course, a new car can be bought using attractive PCPs etc, and you get the exact spec you want, as well as a longer warranty, so I'm not saying it's a bad idea, in fact I bought my previous car (500 hatch) new. This time though I couldn't bring myself to spend all that money on a new one, but I suspect one of the reasons I kept the previous car for 6 years was that it was specced up by me, and was looked after by me from new.
Have fun with that configurator!

Negotiate very hard for a new purchase. The nearly-new prices being offered by dealers for pre-registered examples suggests that there is considerable scope for battering them down on their list prices (provided you're not too choosy on spec).

Well it looks like my panda is in negative equity by about 1k.

I will see if they can make that up - if yes then great. If no, will have to wait a while longer.......