Its normally easier to shorten the cable as any increase in the power level can to controlled by a FPA. Unforetunately increasing that length will reduce the power level and if drops too low will need adjustment at the tap point.
Even if the power level are still within the range with the longer cable I would not recommend a 3rd party cable.
Its £99 or next time the VM service breaks and engineer is needed you can donate some funds towards their drinks..a few drinks 🙂
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M200 Oomph - 5GB SIM + 3 extra SIM family plan - BT whole Wi-Fi - Sky Q
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Don't cry over Virgin price increase instead speak to retentions team and negotiate a better deal.