I am so sorry that you suspect Penny of having wet tail - I too have lost a hamster to wet tail myself so I have my fingers crossed that she doesn't have wet tail - as like you said it isn't a nice way to see them go.
Diarrhoea in older hamsters might also be caused by other things to apart from wet tail- so a vet will be the only person to say for sure if it is or isn't.
One thing I would recommend is giving Penny some probiotic supplements - like this
http://www.petsathome.com/shop/en/pe...robiotic-100gm - which helps to promote friendly bacteria and contains fibre, vitamins and glucose which will help her digestive system and might help to get her poos back to normal and also replenish the nutrients that she losing in her diarrhoea.