Steel

Steel for Flitch beams. Use Mild Steel / Flat Bar. 120mm x 6mm sold up to 6 meter lengths and delivered.

https://www.themetalstore.co.uk/products/mild-steel-suppliers-uk-mild

Could also use www.metals4u.co.uk/

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Steel Frame.

Keith Hussey Steel.
Hardwick Lane,Virginia Water.

178mm x 102mm (7"x2").
A  steel frame for a 5x6m cabin costs Roughly £1,600 +vat