Hello Sam & Jet - welcome to COL
She sounds (and looks!) a beautiful girl
I have a 10 month old boy named Buckley & have found this forum and its members to be an absolute gem in his formative months.
We have walked Jet off the lead since she was 13 weeks and has been brilliant, last few walks have been a bit disheartening. Yesterday, she picked up a plastic bottle and ran around with it and ignored my recall for a good 2 minutes or so..eventually she came to me but i was really mad and upset by then 
i walked her home which was probably the wrong thing to do... she also did the same thing with my daughter who took her to the field with her ball, she would not bring the ball back and kept running away from her. I am sure this is just a phase but things were so good. I have been doing some basic recall with her today and she has been fine, but her behaviour is a little naughty, taking stuff she knows she should have, she has the devil in her at the moment 
Do you have a training lead (also known as a ''long line'') for Jet? I'm currently using one again as we're going through the teenage phase quite like yourself!
If Jet enjoys plastic bottles normally, could you perhaps take one out when you walk her? Buckley recently acquired a plastic bottle whilst out on a walk too, he was also running off & pretending to be deaf so I managed to trade it for a treat & then kept the bottle in my hand which really gained his attention
When he was around 7 months I made the mistake of handing him a brand new (literally bought a few mins previous) Air Kong toy which he completely made off with and towards a road
After we reconvened I decided I'd be the keeper of this toy & he could only play with it if I kept one end of it - which worked to my advantage if he began to wander off as the toy was so highly prized.
I also take squeaky toys out with me, having received COL advice to do so - I can be seen walking about with a furry squeaky duck knotted in my belt loops
What's her favourite toy/s?? Could you take them out of the house & only play with them on walks to gain her attention? I should add that she shouldn't be allowed to play with these by herself as she's likely to consider you'll want the toy & could engage you in a fun game of Chase which you really don't find fun.
Buckley enjoys tuggy games so the squeaky duck is also used as a tug.
Have you trained Jet to the whistle? It's not required at all, it's just that some people find it a very useful recall-enforcer - there's a thread in the Behaviour and Training section of COL that explains recall whistle training.
Any advise on jumping up at guests? We ask guests to ignore her on arrival but as I am sure everyone knows, they never do!!! Will she grow out of this? I really don't want to encourage this.. Our dogs before we retrievers and we never had a problem with this.
Yep no one other than the immediate carers of a dog tend to listen
I suppose it's hard as puppies don't give people reasons to ignore them do they?
My boy does it, but it's lessoning now as he matures. The best thing to do I've found is ignore till all four paws are planted on the floor or if the pup is sitting nicely - but then don't over-fuss the pup as this could start the Jack in the Box routine all over again.
Do you have a baby gate or a room divider? Just wondering if you could practice calm greetings with your family members/friends - have Jet waiting behind the gate & ignore all excitement until she's settled & treat. You can also teach a dog to ''calm down'' with a word - IIRC my puppy book said that when the dog is naturally relaxed and sleepy, run a hand over the body of the dog & in a gentle whisper say your word (e.g. ''Settle'') repeat & repeat until the day comes when you can use the word with the correct calm response....however, I'm sure the writers of the book never studied Cocker Spaniels
We have varying results to ''Calm''....
Best of luck with your little girl, looking forward to hearing and seeing more of her
