As an update, I called BA this morning to see if I can drop the outbound and re-price the ticket into a one-way. The original ticket was fare class "O" out and "V" back, total cost when booked GBP 159 (it was actually around GBP 50 cheaper than booking two one-ways - otherwise I would do that).
BA said dropping the first leg and repricing into a one-way FCO-LGW ticket will cost GBP 389. There would also be a refund of taxes (GBP 40), largely offset by the telephone fee (GBP 30). So this effectively means paying the difference in fare between what was available back in January and what's available now - which doesn't help.
I ended up making the change to the outbound online and keeping the return - an eye-watering GBP 532 (only Y was available - all flights tomorrow book into B or Y now). Work will cover this, as the reason I can't fly out tonight is work related, but that's just a bit crazy...