Some Britpart items are from SPI not all. Rear crossmembers are not SPI and are not brilliant fit. SPI parts are excellent but as of 21-12-08 SPI have ceased manufacturing/trading so unless some other manufacturer buys the tooling off them chassis parts of any quality will become hard to find. There were a lot of parts that SPI made that no one else make such as Defender 1st/2nd row doors, complete front wings, 2002 on one piece rear door, Discovery rear crossmembers, and a lot of Range Rover Classic parts.
Simon