I thought it was 'ego balneo lavo' ??
'ego lavo' is just 'I wash'. And strictly speaking the 'ego' is redundant; you'd often write just 'lavo' for 'I wash'.
To say 'I wash myself' you need the accusative pronoun 'me' as well ('me lavo').
I thought it would be labrum. What case is "labro"?
Thanks - so I take it that the preposition "in" demands an ablative case.
No, not necessarily: "in" takes the ablative for senses like 'in, on, at'; but it could also take the accusative for senses like 'into, toward, against', depending.
When you are talking about a location with no movement, you use the ablative. "I am in the city." -> in urbe sum.
If there is motion 'into' then in takes the accusative. "I go into the city." -> in urbem eo.