You get a free PDF legally. The 2021 edition is copyright-protected. Here are official sources:
The 2nd Edition (2021) replaces the original 2000 edition, bringing the contract in line with modern procurement practices and legal standards. short form of contract 2nd ed -2021 green book- pdf
A major novelty where the Contractor’s overheads for compensable delays are pre-calculated using a formula in the Contract Data . This acts as liquidated damages, avoiding the need for complex expert assessments after a delay occurs. You get a free PDF legally
Why should you choose the 2021 edition over older, free PDFs floating online? The 2nd edition addresses several legal pitfalls of the original. short form of contract 2nd ed -2021 green book- pdf