What would be the import costs for bike bits be for both a $120 and a $180 purchase from the states to here?

Ta

Import on bike bits is 4.9%. Then you’ve got VAT (20%) and handling charge. The latter is a flat rate (£18 or so) so is a lot (as a %age) on a small purchase.

Try here

http://www.dutycalculator.com

Is it still the case that HMRC just ‘sample’ small personal packages rather than levying duty and VAT on them all?

It’s a bit of a lottery but if you factor in at least 25% of the purchase price AND the shipping, anything less is a bonus.

You also have to pay tax on the shipping costs…
Edit.
As stated above.

Recently bought something from US that was in total with shipping £47…got charged £24 in top in import VAT and PF fee; expected to be charged but bit steeper than I was expecting 🙁

I looked into this just over a year ago and total costs were circa one third of the value of the bike. Bike and bike parts carry same charges. Import duty and vat plus another small charge. Duty paid on full amount inc freight as stated above. The costs are collected by the shipping co who will not deliver until the duty is paid. You may be lucky but it was too big a risk for me. Bought from a not so local bike shop.

No import duty under £135, I seem to remember, but you still pay VAT. You’re better off buying both seperately.

EDIT – no import duty on bike parts.