CGT on sale of farm land

If the proceeds from the sale of a part of the farmland is reinvested into demolishing and rebuilding the farmhouse(currently only live in 1/3 as in state of disrepair) is rollover relief available or will capital gains tax be due on the proceeds?

Rollover relief was abolished after 03/12/2002. Unfortunately the CGT will have to be paid.

Separately, I assume all avenues for possible relief from CGT have been pursued, e.g. retirement relief etc.

Graham is correct - rollover relief is gone. As he said, you need to explore all possible options quickly as there are strict payment deadlines as follows:
For disposals in the period 1 Jan to 30 Nov - payment is due by 15 Dec
For disposals in the period 1 Dec to 31 Dec - payment is due by 31 Jan.

If you do not submit an annual return of income (Form 11 or Form 12) you will need to submit a Form CG1.

Penalties, surcharges and interest can be significant if you are late in any of these deadlines.