Not sure if i have made things potentially worse - ventilation of loft.

No, you have not made things worse. Any sort of ventilation is better than none at all. You could add a few tile vents higher up roof Which is an easy install of you felt you needed to.

Sofit vents im my opion shouldnt be the cause of any up lift on the roof but will let a little bit of air throught. if the roof is a traditonal slate roof then usally they wont be felted with a breathable membrain and will be back pointed. But you will find with the heat from inside the house and the cold from the out side will cause the back of the slates to get wet /condensate this over time can also damage the wood work/roof batterns. if your doing a full referb depending on the state of the the roof it may be best to have it completley re done then you know everything is Secure and water tight from the roof down. That been said you can also get tile vents and slate vents that can be fitted to the roof to allow for more Ventilation up if your unsure most roofing companys do free quotes and may be best to have a trusted company come give you some advice hope this helps. Many thanks billy-b newell roofing