To complain or not to complain?

Wash 'em, wear 'em, and put it down to experience.

Then leave a review on the website with the details - you'd be surprised how sometimes this jolts them into action!

Then never use them again.

I would most definitely get those returned, ASAP.

Get them sent back!

On hygiene grounds, the store is supposed  to ensure that anyone who tries them on keeps underpants on (same as socks when trying shoes). I wouldn't accept them.

I'm not exactly squeamish but that is just rank.  I wouldn't expect it on a five pound pair of Decathlon disposables, never mind a hundred quid's worth of Castelli. Name and shame the retailer please.

That's massively disappointing when you've spent a wedge of money. You should ask for a partial refund as the goods are clearly second hand and the price should reflect that.

Everyone is different but I'd rather have sent them back with a letter to tell them how unimpressed I was!

You would certainly get a return no problem- though it doesn't sound like you want the bother,  but you would likely also be offered £10-£20 credit/refund to keep them. I would say you should let them know- its pretty gross and they shouldnt have sent them in that condition.

I work in CS for an online bike seller btw

Under the Distance Selling Regulations you have 14 days from delivery to cancel your order without needing to give a reason, so you are legally covered if you just want to send them back.

Send them back, that's not acceptable. I understand that people may try things on, but I don't want to see any evidence of it.

There's nothing to cover you legally here (I don't think), but they are clearly second hand now and therefore shouldn't have been sold as new. The shop should do something out of good will.