If you have an order for costs that says something along the lines of "respondent to pay the petitioner's costs to be assessed if not agreed", you will have to prepare a bill of costs (detailed assessment).
The bill is a prescribed format - an example of which can be found here:
www.justice.gov.uk/courts/procedure-rule.../new-precedent-a.pdf
If your solicitor's costs were £800 + VAT, you will have to break that figure down into the different areas of the bill ensuring that you do not claim more than you have paid to your solicitor. If the monies paid to your solicitor include other work not relating to the divorce (e.g. finances/children) you should exclude this.
Once you have a bill you need to serve it on your ex/ex's solicitor with form N252 (notice of commencement of assessment of bill of costs) (
formfinder.hmctsformfinder.justice.gov.uk/n252-eng.pdf) AND the order for costs. The bill should be signed by a partner of the firm of solicitor but if they are no longer acting, you can sign it as the petitioner although you will need to amend the certification page to make that clear.
Once served, the respondent has 21 days to file points of dispute AND make an open offer of settlement. You then get the same time to prepare replies to points of dispute which are optional but advisable.
Any point after that you can apply to the court for a provisional assessment of the bill of costs using form N258 (
formfinder.hmctsformfinder.justice.gov.uk/n258-eng.pdf) enclosing the bill, the points of dispute and replies (which should be the same document), a copy of the order for costs and a court fee of £369 (for bills under £15,000).
The court will carry out an assessment and you must then agree the arithmetic of the bill, the order for costs (of the assessment process) and calculation of interest with your ex. The costs of the assessment process should give consideration to any adjustments by the judge and whether the open offer made beats the amount awarded by the court.
If you can't agree those items, you can prepare written submissions within 21 days and the court will decide on paper what is appropriate to pay. At that point you will have a money judgment which can THEN be enforced against your ex.
Charles