Olá, @Barrie. Indo is the gerund (the -ing form) of the verb ir. Gerunds are commonly used in certain regions of Portugal, such as the Alentejo and parts of the Algarve, but they have limited use elsewhere. The most universal way of expressing what would be the Present Continuous in English is the following:
auxiliary verb estar + preposition a + main verb in the infinitive
Some examples:
Eu estou a chegar. (I’m arriving)
Elas estão a ir à praia. (They’re going to the beach)
Tu estás a ir para casa? (Are you going home?)
If we switched to the gerund instead of using the infinitive, the structure would be:
auxiliary verb estar + main verb in the gerund
Eu estou chegando. (I’m arriving)
Elas estão indo à praia. (They’re going to the beach)
Tu estás indo para casa? (Are you going home?)
So, as you see, the gerund doesn’t replace a conjugated verb, but a verb in the infinitive. This is to say that in this Present Continuous structure, you can’t use “indo” instead of “vou”, but you can use it to replace “[estou] a ir”.