The Roman Map of Britain Iacc[i]odurum? or Lactodurum? Towcester, Northamptonshire 

Iacio Dulma (R&C 95) next
Lactodoro (AI 4706 Iter II)
Lactodoro (AI 47611 Iter vi)

    I suspect we have a river-name based on Brit *iekti-, W iaith 'language' as the first element. Recognized examples of *Iectona 'talking one' are the Welsh Afon Ieithon/River Ithon (Radnor) Ieithion Black Book of Carmarthen, the Scottish River Ythan (Aberdeenshire) aqua de Ethoyn 1373, and the Oxfordshire Ray earlier recorded as Geht ca.848 (see ERN Ight p209).  *Iecto- would represent the British name of the River Tove. 
    The Tove rises near Sulgrave SP5545 Sulgrave 1086 OE sulh-grf 'gully-grove', but compare Sulloniacis on the Silk. Nearby is Eydon SP5450 Egedone 1086 'ga's hill'. Eydon is part of a different catchment. Adstone SP5951 is within the Tove catchment. It is recorded as A- Etenestone 1086, Attelestuna -ton 1154x89-1356, Atteneston 12th-1330.
    The second element is duro, with the apparent meaning of ' low-lying fortified site'. The -lm- of the Cosmography's-dulma is the result of a superscript r which split into - l ~-, yielding -lm- when expanded.

Towcester SP6948 Roman Town

    Next Towcester is Heathencote SP7147 Heymindecot 1220, Hekemundecot, Hemundescot 1236, Heghmundecotes 1307; considered 'Hahmund's cot(s)' by Ekwall. Most of the forms look like OE a(ge)mt 'river meeting', as in Eamont. Oddly, the modern Heathen- more resembles Ieithon/Ithon than the earlier forms. I am told that the name is pronounced 'Heavencote'.

   Another possibility for the first element as a river-name is Brit *ikko-, OIr hcc 'healing', W iach 'healthy', Corn yagh, Bret iac'h.

    I was asked from where the H of Heathencote came, and I couldn't answer. But later I came upon the name Hensall N Yorks SE5923, which CDEPN has as Edeshale 1086, Hethensale 12th, Hethens- 13th-1379, Heuens- 13th, 1364, le Henssall 1406, Hensall from 1444. Here an H is developed, and remarkably the form Heuens-, a dialectic substitution of [v] for [].

    I'm afraid that trying to find a remnant of *iecto- has been ultimately unproductive. Unless this area remained as a pocket of British speakers later than what would be expected the sound change of -ct- to -th- would be impossible. I still prefer the form *Iecto- to Lacto-, and note that the entry preceding it is recorded as Lectoceto (for Letoceto) - ample opportunity for cross-contamination. The river-name in question is not necessarily that of the Tove. Silverstone Brook would serve well as the locative river.

   Taking another look at Brit *iekko-, Matosovic has it as Proto-Celtic *kk-, *iakk- 'cure, treatment, salvation'. The early name would be the same as that postulated for Itchen (see CDEPN p 334). The Cosmography's entry for the river Itchen is Axium for Iaxium/Iaccium.
If Iacc- was by then Iac-, it would be very similar to OE
Hah- (OE h = c). That -mund represents OE -mt 'meeting' is uncertain, but Silverstone Brook joins the Tove at Heathencote. Another remaining question regarding the form Iacio-, does it represent Iacco- or a scribally simplified Iaccio- as indicated by [I]Axium? Lactodoro would suggest Iacco-.