Ideally this will have been carried out when you got the appraisal done on the diamond, but if not, you will need to get a valuation done in order to obtain insurance for the ring.
The Guild of Valuers and Jewellers www.gvj.org.uk has a list of jewellers who are able to supply valuations on diamond engagement rings. Enter your postcode into their jeweller search feature to find one that is local to you.
It is highly recommended that you take some high quality photos of your ring as this will help if you ever need to make an insurance claim for it. It may also be a prerequisite before some insurance companies will insure it.
A decent quality digital camera will be fine for this.
