Hi,
I have rented a fair number of cars from carjet.co.uk. They are a broker, probably like the outfit Sil referred you to. I've never had a problem and always find that their prices are well below other quotes.
But two points to consider: one way rentals will add a chunk to the price, but since Burgos and Leon aren't too far from each other, it might not be that much, and might be ultimately less than having everyone take public transit from Burgos to Leon after a drop-off.
If you are from the US, though, (I assume you're not since you say "car hire" but maybe others who are reading this are) and if you rely on your credit card to supply the CDW, you should know that the brokers I have dealt with in Spain all give you a rental price with CDW included, and they cannot take it out. And then that means that your credit card will not give you the protection, because rule #1 for them is that you must waive CDW offered by the car rental company. I have been told that Auto Europe will give you the option to waive the CDW. This CDW thing is, I think, extremely confusing, but the main thing is that if you're relying on your credit card to provide CDW in case of accident or damage, you need to look very carefully at what your rental price includes.
Sorry, I know I am drifting off topic, but this is one of those things that I've never really paid attention to before and have just learned that I've been really lucky that nothing has ever happened during a rental, because my credit card would not have covered the damage.
I'm glad that in a few weeks I'll be walking and not driving! Laurie