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Convention in the AIP, if there is any doubt, is use the first one listed - in this case 126.35. Bottlang also list it as the primary approach frequency for the airfield.
It is however a poorly worded entry - look at any others and you'll see there is no doubt. Anyone from Manston care to sort out their entry in the AIP?
As for flying into other airfields with more than one approach freq. Again - use the 1st one listed unless you're told otherwise when (if) handed over inbound.
DOC - Designated Operating Coverage. The maximum range and altitude at which you can talk to another unit without (in normal atmospheric conditions) interfering with another station with the same frequency. (In the absence of an ATIS some commercial carriers call TWR frequencies for the weather when over 100 miles out up at cruising alt - FL245 and above. They shouldn't, and if they do then they're normally told where to go
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