Usually mortgage lenders (for the buyer) demand that searches are under 3 months old on completion.
There is a search indemnity insurance that can be put in place to cover the gap though and it usually costs about �25.00.
I'd say it would be fair, if the buyer's solicitor raises an issue over any out of date searches, to provide an allowance on completion for the indemnity premium providing you don't know of any issues which may have changed on the searches (eg the property has been served with a council notice etc...).
Usually it's a local search (planning and other matters), water and drainage, sometimes environmental though they don't tend to come in a HIP and coal depending on the area.
Make sure you have a few colour copies of the HIP (there will be colour plans in there) and if it's online only make sure that the weblink isn't taken down.