As the other replies have indicated, I would suggest a local land agent carries out the end of year valuation.
Fluctuations in livestock valuations can have large tax implications - upwards and downwards and livestock markets can be pretty volatile.
By the sounds of it you have a very diverse farm with distinct livestock enterprises and each will be at a different point in the youngstock lifecycle at your year end. For example your cattle could be finishing on farm for 18 months or more with a gestation of 11 months, pigs potentially 3 litters over 2 years so several different ages of youngstock and sheep lambing just once a year and depending when lambing season is for that farm you may or may not have lambs at foot or in store. I am an agricultural bookkeeper of thirty years and I would not attempt to value this stock myself, its one for the professionals I would suggest. A single suckler herd or a flock of sheep, yes, but not this.
Perhaps the farm accountant can advise you of the history?
Good luck with your new client!