If you are an employee and feel that your case is strong enough to justify compensation under section 40(2) of the Patents Act 1977. You will need to explain in the particulars of claim who you are, why you were employed and what your invention entitled by citing the patent number, when it was granted, the filing and publication date.
You will then need to set out when the defendant commenced sales of the particular products falling within the claims of the patent. How the defendant became aware of the idea, ie did you mention it to him by way of written description and sketches.
As a consequence you sold ie assigned to the Defendant all rights in the invention and pursuant to the assignment agreement you as the claimant and defendant agreed that the defendant was to pay you a lump sum per year for 20 years from the date of filing of the patent.
You will need to claim that the basis of compensation is, in accordance with section 41(1) of the Patents Act 1977, being a fair share of the defendant’s benefit and that the fair share is to be calculated on the basis of an arm’s length licence or, alternatively on the basis of a reasonable royalty.
This may be a matter for expert evidence, but do give an estimate ie a payment of the order of 5-10% of gross sales by the defendant- and good luck!