If your computer is a desktop you may find a 'line in' jack socket at the back into your mother board or soundcard that may work a lot better than the mic input. If there one is it may not be active and a trip into Windows Control Panel may be required to configure it.
Otherwise, if you have a Windows or Mac device something along the lines of a Behringer U-Control UCA 222 would be a relatively inexpensive device that is Audacity compatible (they provide a link on the downloads page for the device). You would need a suitable 3.5mm jack to 2x RCA cable to join the two. Generally this will produce a much better result that using the microphone input on a computer.
There are a number of similar devices, primarily designed for music transfer, but this one looks as if it would be OK. I use something similar but it costs a lot more and would be overkill for your needs.
A link so that you can read about it is
http://www.behringer.com/EN/Products/UCA222.aspx go to downloads and the full manual is available below the pictures. The overview on the product page includes your requirement as one of the devices uses, which inspires confidence.
This device is easily available in the UK from many sources including Amazon, which may not be a lot of use to you, depending on how you source your kit.
The Amazon reviews may be helpful in understanding its uses, don't be put off by all the musical jargon.
There are many cheaper devices that will not improve the quality of your transfer and many others which cost about the same or more as the Behringer, that will, I have just used the UCA222 as an example.
There are a number of tape to USB convertor devices available, read the Amazon reviews, but I would tend to use my own recorder with an external soundcard device such as the UCA 222 for the best chance of preserving the quality of the recording. If nothing else the alignment of your tape heads will match your tape recordings better.
Certainly follow the suggestions of Mono before committing any cash on anything else, there is no point in spending money if you don't have to, but unfortunately many PC mic inputs are generally pretty terrible, not all, but most.