Fogot to reply to you last night lablover. If I try to keep holding her jaw, she starts to try to throw her head around - and if I cannot keep hold, yes she spits it out.
Friend of mine had this problem as well with a lot of mouthing thrown in for good measure. The recomendation to them was to get the dogs nose up so it was harder to spit the item out. For this particular dog it was done by scratching between the eyes as he pushed into it and while he was concentrating on the scratch he forgot to mouth and or spit it out so could be rewarded - the original suggestion was to scratch under the jaw as you could easily position your hand to grab the dumbell if they did try and spit it out from there.
You did mention the fun the dog had with the soft toy so maybe it would be worth getting him to understand the exercise with something he doesn't mind hanging on to - a rolled and taped towel for example. Once he understands the principle of what you are asking it should hopefully then be easy enough to transfer across to the more formal item.