So, am I right in thinking that with these battery optimisers, I can also buy an adapter to enable me to charge a phone?
Depends what you mean!
If you mean to charge the phone with the Optimate, no! But why would you as if you have an Optimate you also have a mains plug
There are numerous suppliers selling stuff that will connect to the fly-lead from the battery. I use a lead (to charge my camera batteries in my tank bag as well as the phone) with a car-style cigar lighter socket on the end which accepts a multi-USB charger adaptor. Simples!
I have always used Elite...
http://www.elitemotorcycleservices.co.uk/optiacc.html...who know what they are talking about and can supply almost anything to do with an Optimate.
Other suppliers etc. etc. etc.