Your Dyson DC01, regardless of variant, is fitted with two filters. One is located in a cage slotted into the body of the machine and the other is located under a flap below the cyclone unit.
It is important to pay regular attention to your filters as they allow your Dyson to breathe. Depending on usage, they should be checked every three to six months. You should pay particular attention to your filters if you have had building work carried out. Your Dyson does not like plaster dust.
Most Dyson DC01’s were originally fitted with disposable fibre filters (a few very late machines had washable filters). They are quite inexpensive to buy. You can pick up a pack of eight standard filters (enough for 4 filter changes) for less than £5 here: Dyson DC01 filters
It only takes literally moments to change your filters. It is not necessary to take it to a vacuum cleaner repair shop. Anyone can do this at home with no expertise and no tools.
Look on the base of your machine. You will see a cage that just pulls out like this:
When you have pulled it out, it simply hinges open to swap the filter like this:
Snap the cage back together, and simply slide it back into the machine. That’s the first one done.
For the second one, press the tab on the bottom of your DC01 to allow your dust canister to swing forwards and upwards (same as when you are emptying it):
Whilst holding the cyclone unit out of the way, lift up the hinged flap like this:
The machine pictured here is an “DC01 Absolute +” model which is fitted with a circular HEPA filter as you can see. To change this filter, it simply lifts out of the circular hole it sits in, and the new one slots back in. Important: These circular HEPA filters ARE NOT washable as some people think. If yours is past its best, simply replace it with a new one. They are less than £10.
If your machine is a standard machine, instead of this circular HEPA filter, it will have another pad identical to the one you changed first. Simply swap the new for the old and you are good to go.
These DC01 machines — despite still being brilliant — are a little elderly now. Occasionally, the hinge lugs or the cyclone retaining clip on the bottom flap will have snapped. The filter cages can break up too over time. You can replace the cage or the flap very inexpensively. You can buy genuine Dyson recycled parts and a host of other new filters and parts for your DC01 from here: Dyson DC01 Spare Parts