I've been researching cheaper ways to get into resin infusion without having to splash out on an expensive vacuum pump.

Would a manual pump like this work?:

https://www.ebay.co.uk/itm/Engine-Oil-Brake-Fluid-Extractor-Removal-9L-Sump-Transfer-Vacuum-Hand-Pump/172659264201?epid=2164769182&hash=item28334b4ac9:g:I5MAAOSwBt5ZD-Xf

Some people who make RC planes use hand brake bleeder kits like this one:

https://www.ebay.co.uk/itm/HAND-VACUUM-PUMP-HELD-BRAKE-BLEEDER-TESTER-SET-BLEED-CAR-MOTORBIKE-BLEEDING-KIT/391718817400?epid=595953556&hash=item5b3442ce78:g:c48AAOSwB-1YuE1W

An interesting idea

It feels like the brake test equipment might offer the best fittings etc. to obtain a good seal but, I'm guessing, that it'd take ages on anything but a small mould to extract the air. The internal volume of a brake system would be very small, I would have thought. I'm not certain that the fluid transfer system would provide the necessary air-tight system to make it a worthwhile consideration.

You can infuse parts without having a pump constantly running during the infusion, but you're like to get voids and evidence of air still within the part once cured. I have, on a couple of occasions, neglected to open the valve on the pump when starting to infuse and although the part did infuse okay, it was evident that the usual de-gassing of the resin during the process had not happened which left a less than perfect end result. That said, it was surprisingly good, considering.

Another option to consider might also be the vacuum bags used for the storage of clothes and duvets. These are pulled down using a vacuum cleaner and are pretty cheap and widely available via e-Bay.

A little risky if you struggle to get a perfect bag seal, but if your intended part is a one-off and you're prepared to accept a bit of trial and error and the likelihood of less than perfect cosmetic results then maybe worth a go? It might also be more productive to think about wet-laying your part and just using the alternative method chosen to vacuum bag afterwards. At least you might get a bit of benefit with improved consolidation.

I, for one, would be interested to see what kind of results you can achieve though with whatever method you try out.

I totally recommend buying a cheap rotary vane pump. The two options you linked are -at least in my opinion- not suited for vacuum infusion. The first one does not show an achievable end pressure (but the construction look like it might be around 800mbar (like a vacuum cleaner)). And the second item only achieves about 200mbar. For infusion you are looking for a "actual vacuum", not a "slight underpressure". We are talking of at least 10mbar absolute pressure. This is about 1/100 atmospheric pressure.
An other point is, that your volumetric flow rate will not be high enough, to detect any leaks in your bag, as the slightest leak will let more air in, as you could possibly pump of. And doing anything about the leaks might be tricky while pumping.
The cheapest new rotary vane pumps cost 80£. If you are going into vacuum infusion, it is definitely worth saving a month for the extra money.

Thanks for the comments guys. I guess it's because what I want to do will be a one-off so it's seems expensive to shell out on all the equipment. I find it hard to find secondhand equipment, there doesn't seem to be a lot about. I have tried the vacuum bag idea for clothing, but they are very easy to tear and kept leaking.

Infusion is obviously not the ideal method for one off projects.
Keep in mind, that the costs of a failed part easily exceeds the difference to a cheap pump. Especially if you sell it afterwards.