Surely all I can glean from the anova summary is whether the accept or reject the null? Because the analysis is run on rank-order data, any post-hoc analysis will not tell me anything about the data in the original units of measurement. I' be grateful of others' views or advice.

Is this what you did?

https://www.ibm.com/support/pages/can-spss-do-nonparametric-or-rank-analysis-covariance-quades-test

Why did you move to using ranks instead of the original data in the first place?

p.s. Maybe ordinal regression would be helpful here.

Analysis of covariance is used to test the main and interaction effects of categorical variables on a continuous dependent variable, controlling for the effects of selected other continuous variables, which co-vary with the dependent.