I believe it to be a mess up by the Planning Dept in saying they will hold it by abeyance. The Planning Inspectorate appeals form makes the point that any appeal must be received before any Enforcement Date imposed by the Planning Dept. You have done this.
http://www.pcs.planningportal.gov.uk/pcsportal /pf03form.asp
I'd just phone them to remind them that you have an Appeal in progress. I think they will say, we know - we're just following our planning process through with an enforcement date but awaiting the outcome of your appeal.
In the event that you lose the appeal, you then have a fixed short time in which to comply.
If they don't do it this way, if you lose the appeal against the planning decision, you could then theoretically lodge another appeal against the enforcement notice. They are forcing the Inspectorate to make a decision against both the planning decision and the enforcement notice in one bite.