Conditional vlookup/index with if formula

I am trying to build a spreadsheet that will help me save time with my job. I have attached the spreadsheet that I am working on.

I currently have a spreadsheet with 2 tabs - Costings, PipeOD. Sheet 'Costings' has 4 columns (A - D). Sheet 'PipeOD' has the data that I want columns C and D to automatically recognise depending on the input in columns A and B of 'Costings'.

Currently I have used an index formula to automatically recognises whatever is input in column A. This searches columns A and B of 'PipeOD' and inputs the value in column B in column C of 'Costings'.

I cannot build a formula that will enable the same kind of searching of whatever is in a cell in column b of 'Costings' and against C to G of 'PipeOD', with entry of the data in column G.

Example: In costings I want to enter Pipe OD as 1/8", this returns 10.3mm as the result of the index formula. Similarly if I entered 10.3 it would also return 10.3mm as the result. In 'Costings' I also enter 10S as the wall thickness. This should return 1.24mm in column D as per the data in 'PipeOD'. The problem I am having is that the formula first needs to search the Pipe OD size and then search columns C to G returning the value in column G for the row relevant to both the Pipe OD and the WT in columns A and B of 'Costings'.

I know this was a bit of a long-winded explanation but please help!