here's some background reading for you
MNPSA
UK CAP694
http://www.caa.co.uk/docs/33/CAP%20694.pdf
Since Shanwick is outside the IFPS zone, ensure your flight plan is addressed correctly using the "AD" line on the flight plan
Shanwick AFTN Address (EGGXZOZX). Santa Maria address (LPPOZOZX) when sending your flight plan to eurocontrol for processing
rab-k is correct for santa maria NAVIX estimate, requested flight level and mach number and maximum acceptable level.
Requests for oceanic clearance northbound on T16 via NAVIX will be made to Santa maria on HF frequencies 3016,5598 8906 (depending on the frequencies they are using)
requests for clearance northbound on T9 (via BEGAS) will also be via HF shanwick in this case, southbound - use VHF