Ceolbehrt was a Mercian Bishop for a frankly poorly attested period in roughly the middle of the 9th Century. According to John Stow's 1598 Survey of London (specifically his Gouernors of the Citie of London, and first of the Ecclesiasticall, Bishops, and other Ministers there) he was Bishop of London in the 830s, between an Æthelnođ and a Caulfe. Ceolbehrt is poorly attested, appearing only once as Ceolberht episcopus in a charter of Wiglaf's dated to 836 (S190), and not being mentioned at all in the Anglo-Saxon Chronicle.His predecessor's predecessor seems to have been Bishop Osmund, who appears in a charter of Wiglaf of Mercia in 833, while Ceolberht doesn't appear to have attested any charters during the reign of Beorhtwulf of Mercia, so he may have already passed out of office by 840. Certainly Stow places Caulfe as being in office by 841.

It's unfortunately not particularly uncommon for Early Medieval bishops or even archbishops to be fairly poorly attested, and Ceolberht seems to have been office for perhaps 4 or 5 years at best.

Ceolwulf II is entirely unrelated to Ceolberht, beyond the two being Mercian. Ceolwulf took the throne of Mercia in 874 after Burgred was deposed by the Danes. Unlike the centralised dynastic succession of the Cerdicings in Wessex, Mercia functioned as more of a 'tribal' hegemony in which kingship cycled between a number of pseudo-'tribal' dynasties. Based largely on naming conventions, Ceolwulf II is commonly considered to be a member of the same dynasty that had produced the previous Mercian kings Coenwulf and Ceolwulf I. Given that the major literary source for the period is essentially Cerdicing hagiography, it is perhaps not surprising that Ceolwulf II isn't particularly well attested, and that much of what is written about him is demonstrably wrong. The Anglo-Saxon Chronicle describes Ceolwulf as a "foolish king's thegn" installed as a puppet ruler by the Danes, but this is belied by numismatic and Welsh textual sources. The Welsh histories of the Annales Cambriae and the Brut y Tywysogion instead suggest that Ceolwulf likely managed to fight the Danes to a negotiated peace settlement like a proto-Danelaw. Certainly for a 'puppet' he seems to have spent a great deal of his reign campaigning aggressively and successfully to expand Mercian holdings and overlordship in Wales. Numismatic evidence also suggests that he and Alfred of Wessex were in fact close allies. Alfred certainly didn't intercede to prevent Ceolwulf's expansion into Wales despite entreaties from various Welsh rulers, and Alfred also used the London mint to manufacture a (previously considered rare but now known to be quite extensive) issue of pennies with a design based on a Roman "Two Emperors" solidus depicting him and Ceolwulf as equals and allies.

Ceolwulf was likely killed in Wales in 881. He simply disappears from the Anglo-Saxon Chronicle but the Annales Cambriae record that in 881 the Welsh victory over the Anglii (the Mercians commonly being thought of as Angles as opposed to the Saxonibus of Wessex) achieved "God's vengeance" for Rhodri ap Merfyn, the Welsh king killed in battle with the Mercians in 878. Thomas Charles-Edwards argues that the victory at Conwy was over Æthelred of Mercia rather than Ceolwulf, but this is based on a 13th Century source, and 'God's vengeance' for the killing of a Welsh king certainly suggests that an English king might have been killed, conveniently at a time when Ceolwulf II drops out of the historical record.

You might be interested in exploring the AMA panel we did on AC:Valhalla here.