If i setup a trial subscription like say £1 for 7 days full membership can this automatically change to 97 a month after the initial 1 week expires?
Yes, each subscription plan can have an initial duration and price as well as regular duration and price. Initial duration and price is of course optional. Once the initial duration is over the user will be asked to pay for the regular duration and price; it is NOT legal to automatically charge the user the new price/duration so they must (when asked) "renew" their subscription to the full price.
Also if i have a silver member paying me 47 a month, how could they upgrade to gold which unlocks more of site and costs 97 a month?
All you need is 2 subscription plans and if configured correctly the user simply upgrades to the better plan. This can be setup a number of ways with either a parent plan and 2 child plans or 2 parent plans that are exclusive or non-exclusive, etc.. just depends on which you prefer.