As above, we have a custom style of Organisation (Adviser) that can be can be linked to opportunities via a lookup function on the opportunity itself. Currently in the adviser org we have a manual field to enter 'number of opps generated', i want to change this so it will be a count of all Opps which have are attached to the Adviser. Am i correct in thinking it will be a rollup function? I could do this on excel pretty easily but i am new to salesforce . . Any help appreciated, Thank you
You cannot use a roll-up summary field with a lookup relationship. You will need to have some code (most likely a trigger) that counts the total number of opportunities related to the adviser.
Roll-up summary fields are only available on master-detail relationship types. Abhinav Guptas has a pretty handy solution for this that you can use for development (or can ask a developer to utilise) and there is another similar solution which you can setup and use from here.