I have a three and a half year old cocker spaniel (working strain), he is as daft as a brush and very friendly towards everyone, never aggressive. Unfortunately he is giving me cause for concern. If I, or anyone else for that matter, play with him using his "tugger", he is quite happy to drag you around the house, growling (in a playful manner, tail wagging etc) with no problems. However, he has started to nose around in bins in the house and if he takes something out of a bin, say a piece of discarded paper for instance, and you try to take it from him, he goes balistic! He has accepted me as leader and although he will growl aggresively (no wagging of the tail!) he will let go of whatever he has inside his mouth. Anyone else tries to do it and it is a different kettle of fish, he becomes really aggressive to the point that I believe he would take your hand off rather than give up what he has secreted in his mouth. Any suggestions for this behaviour? 

Try replacing what he has in the bin, for his favourite toy, ie, his tugger or a treat. Teach him that if he lets go of what you want then he gets something even nicer as a reward.

Personally, I would move the bins out of his reach :D

I'd do both - management can help (removing the bin) but what happens when he picks something up outside?

I have always taught my puppies the command ' leave ' and I have always found this to work for me. I do not know if the OP has used this in training before. :D

Hi Roly, Your spaniel is doing what is called 'resource guarding'.  Like digger identified this can be a huge problem.

You may not realise it but by taking away the object in his mouth, you are actually making the problem worse.  The dog believes it has a high-value resource which it believes others also want.  By taking it from him, you are confirming this belief and saying 'yes, that is such high value that I want it too'.  This will just make him guard it even more in future. 

You should get and read a book which I highly recommend called 'Mine!  A Practical Guide to Resource Guarding in Dogs' by Jean Donaldson, available from Crosskeys publishing.  http://www.crosskeysbooks.com/product_info.php?products_id=451

This book is worth every penny and contains a series of detailed exercises to help eliminate resource guarding.

By the way - the two kinds of growling are very different - tuggy growling is acceptable, not aggressive, just playing.  Growling and not letting you have something is worrying, aggressive, and should be sorted.