Panda specific first choice has to be shop4parts. Failing that find a decent independent local motor factors although they're a dying breed. After that EuroCarParts (I find them a touch easier to deal with than GSF) and for some things even try the local main dealer, they can occasionally surprise you.
Yes shop4parts. A family run firm with real service and cheeffyl help every time so far. Icant speak highly enough of them. Delivery isgreased lightening quick too. They do genuine or OEM QUALITY parts and good prices. I buy batteries from Halfords as they price match honestly so genuinely cant be beaten on price. Theirs are Yuasa which Ive mainly used since 1973. Without issue. I did stray to Exide and regretted it. The warranty they offer is bomb proof and they are local. I dont like the company from other things I have heard I find GSF stuff generally cheap and nasty ditto ECP. But do buy from them as they are cheap, they are available and service is generally OK.
Another "strongly recommended" for Shop4parts from me. Very helpful and easy to speak to staff who are not at all "pushy". I've never been supplied with something from them which, despite having some unheard of brand names, was not of first class quality. Only downside is you need to spend at least enough to qualify for free postage (and invoke the forum discount) for it to be financially worth it.
I also very strongly endorse using local motor factors, although, as mentioned above, independent, often family owned, ones are being swallowed up by the "big boys" (as has happened with my local who is now part of the massive NAPA setup.) I'm still getting good prices and quality of goods from them though. GSF? There's one miles away over in Glasgow, never used them. ECP? Years ago they had a name, for a while, of not having the brand advertised and pushing you onto something less well known. May have changed now? Doesn't really affect me though as their nearest branch is about three times as far away as my two nearest local trade factors so I just have no contact with them.
Halfords for batteries - and a range of other service parts and sundries. Yes, especially when you're really toiling at the weekend when all the trade sources are closed. I find both my nearest Halfords shops very helpful and buy Yuassa batteries from them. However I do have one of their trade cards which makes buying Halfords branded and other specific items, much cheaper. I've never used their workshop facilities and, from what I've heard, I think that side of their business is run much along the lines of other "faster fit" "Drop in" setups. I won't comment further.
Main Dealer stores dept? Been surprised more than once when contacting them for parts prices. When I say surprised I mean both ways. Sometimes much cheaper than expected and on other occasions, so expensive it almost made me weep! Well worth a try though because when it's worth it, well, then it's worth it!