driver,
DCS has two frequencies. The usual one is 135.530(used to be 135.525 but its just changed to become eu friendly) but DCS sector can be split when its particularly busy with transatlantic traffic cutting across from NATEB routing to MIMKU, IBROD..etc then 135.850 will be used for the DCS north sector. The vertical limits of DCS are FL260+. As has been mentioned, the 260 LVL NATEB restriction is designed to keep you out of the DCS sector and will generally only be cancelled late in the evening when things get quieter.
with regards to Galloway 124.825, and Talla 126.3, the country is basically split down the middle with a line joining cumbernauld and a position halfway between DCS and MARGO. East of this line is Talla sector working the inbound aircraft from the south and overflights FL250- , west of this line is the Galloway sector who work the departures from PF and PH departing through DCS and SUBUK and also those departures going through TRN to the southwest and ireland.
hope this helps